thought experiment wiki

Thought Experiments

Thought experiments are devices of the imagination used to investigate the nature of things. We need only list a few of the well-known thought experiments to be reminded of their enormous influence and importance in the sciences: Newton's bucket, Maxwell's demon, Einstein's elevator, Heisenberg's gamma-ray microscope, Schrödinger's cat. The same can be said for their importance in philosophy. Much of ethics, philosophy of language, and philosophy of mind is based firmly on the results of thought experiments. Again, a short list makes this evident: Thompson's violinist, Searle's Chinese room, Putnam's twin earth, Parfit's people who split like an amoeba. The 17th century saw some of its most brilliant practitioners in Galileo, Descartes, Newton, and Leibniz. And in our own time, the creation of quantum mechanics and relativity are almost unthinkable without the crucial role played by thought experiments. Contemporary philosophy, even more than the sciences, would be severely impoverished without them.

1. Examples of Thought Experiments

2. objections to thought experiments, 3. types of thought experiments, 4. some recent views on thought experiments, bibliography, other internet resources, related entries.

Among scientists, Galileo and Einstein were, arguably, the most impressive thought experimenters, but they were by no means the first. Thought experiments existed throughout the middle ages, and can be found in antiquity, too. One of the most beautiful early examples (in Lucretius, De Rerum Natura ) attempts to show that space is infinite: If there is a purported boundary to the universe, we can toss a spear at it. If the spear flies through, it isn't a boundary after all; if the spear bounces back, then there must be something beyond the supposed edge of space, a cosmic wall that stopped the spear, a wall that is itself in space. Either way, there is no edge of the universe; space is infinite.

This example nicely illustrates many of the common features of thought experiments: We visualize some situation; we carry out an operation; we see what happens. It also illustrates their fallibility. In this case we've learned how to conceptualize space so that it is both finite and unbounded. Consider a circle, which is a one dimensional space: As we move around, there is no edge, but it is nevertheless finite. The universe might be a many-dimensional version.

Often a real experiment that is the analogue of a thought experiment is impossible for physical, technological, or financial reasons; but this needn't be a defining condition of thought experiments. The main point is that we seem able to get a grip on nature just by thinking, and therein lies the great interest for philosophy. How is it possible to learn apparently new things about nature without new empirical data?

Ernst Mach did a great deal to popularize the idea of a Gedankenexperiment . He also popularized the term, but he was not the first to use it. That honour seems to go to Georg Lichtenberg, writing about a century earlier (Schildknecht 1990). Mach developed an interesting empiricist view in his classic, The Science of Mechanics . We possess, he says, a great store of "instinctive knowledge" picked up from experience. Some of this is from actual experience and some we have inherited through the evolutionary process, thanks to the experience of our ancestors. This knowledge needn't be articulated at all, but comes to the fore when we encounter certain situations. One of his favourite examples is due to Simon Stevin. When a chain is draped over a double frictionless plane, as in Fig. 2a, how will it move? Add some links as in Fig. 2b. Now it is obvious. The initial setup must have been in static equilibrium. Otherwise, we would have a perpetual motion machine; and according to our experience-based "instinctive knowledge", says Mach, this is impossible.

Figure 2(a) and 2(b) “How will it move?”

Judith Thompson provided one of the most striking and effective thought experiments in the moral realm. Her example is aimed at a popular anti-abortion argument that goes something like this: The fetus is an innocent person with a right to life. Abortion results in the death of a fetus. Therefore, abortion is morally wrong. In her thought experiment we are asked to imagine a famous violinist falling into a coma. The society of music lovers determines from medical records that you and you alone can save the violinist's life by being hooked up to him for nine months. The music lovers break into your home while you are asleep and hook the unconscious (and unknowing, hence innocent) violinist to you. You may want to unhook him, but you are then faced with this argument put forward by the music lovers: The violinist is an innocent person with a right to life. Unhooking him will result in his death. Therefore, unhooking him is morally wrong.

However, the argument does not seem convincing in this case. You would be very generous to remain attached and in bed for nine months, but you are not morally obliged to do so. The parallel with the abortion case is evident. The thought experiment is effective in distinguishing two concepts that had previously been run together: “right to life” and “right to what is needed to sustain life.” The fetus and the violinist may each have the former, but it is not evident that either has the latter. The upshot is that even if the fetus has a right to life (which Thompson does not believe but allows for the sake of the argument), it may still be morally permissible to abort. Theorizing about thought experiments usually turns on the details or the patterns of specific cases. Familiarity with a wide range of examples is crucial for commentators. Most discussions of thought experiments include several illustrations (e.g., Brown 1991, Horowitz and Massey 1991, and Sorenson 1992). There are also two recent books devoted mainly to the presentation of brief, non-technical examples, Cohen 2005 and Tittle 2005. Some special examples with very nice animations can be found at John Norton's website (see below).

Of course, particular thought experiments have been contested. But for the most part, thought experimenting in the sciences has been cheerfully accepted. The great historian of physics, Pierre Duhem, is almost alone in his condemnation. A thought experiment is no substitute for a real experiment, he claimed, and should be forbidden in science. However, in view of the important role of actual thought experiments in the history of physics — from Galileo's falling bodies, to Newton's bucket, to Einstein's elevator — it is unlikely that anyone will feel or should feel much sympathy for Duhem's strictures.

Philosophers are more critical. They worry, with some justice, about how reliable our intuitions really are. Can we trust them in bizarre situations? Kathleen Wilkes, for instance, was very distrustful of Parfit's people splitting like an amoeba. She declared that we simply don't know what to say when thinking about this sort of thing. She declared that a thought experiment should not violate what we take to be the laws of nature. This would rule out Parfit's examples. But such a proposal seems much too strong. We learn a great deal about the world and our theories when we wonder, for instance, what would have happened after the big bang, if the law of gravity had been an inverse cube law instead of an inverse square. Would stars have failed to form? Reasoning about such a scenario is perfectly coherent and very instructive, even though it violates a law of nature.

There are other objections, too. Jonathan Dancy thinks thought experiments in ethics are circular. Daniel Dennett thinks they use folk concepts, so they are inevitably conservative. These objections can likely be met, but they illustrate an ongoing debate.

There are many ways of classifying thought experiments: science vs philosophy, or normative (moral or epistemic) vs factual, and so on. I will outline a taxonomy here based on how they function as evidence. The main division is constructive vs destructive, that is, a thought experiment might be used positively to establish a theory or it might be used negatively to undermine a theory. Each of these is subject to further divisions.

Thought experiments are used negatively in a number of different ways. The simplest of these is to draw out a contradiction in a theory, thereby refuting it. A second way is to show that the theory in question is in conflict with other beliefs that we hold. Schrödinger's cat, for instance, does not show that quantum theory (as interpreted by Bohr) is internally inconsistent. Rather it shows that it is conflict with some very powerful common sense beliefs we have about macro-sized objects such as cats. The bizarreness of superpositions in the atomic world is worrisome enough, says Schrödinger, but when it implies that same bizarreness at an everyday level, it is intolerable.

There is a third type that, in effect, undermines a central assuption or premiss of a thought experiment. Thompson showed that "right to life" and "right to what is needed to sustain life" had been run together. When distinguished, the argument against abortion is undermined. A fourth type of negative thought experiment is quite a bit more complex. I will call these counter thought experiments. Mach produced one against Newton and Dennett produced another against Jackson. Newton offered a pair of thought experiments as evidence for absolute space. One was the bucket with water climbing the wall, the other was a pair of spheres joined by a cord that maintained its tension in otherwise empty space. The explanation for these phenomena, said Newton, is absolute space: the bucket and the joined spheres are rotating with respect to space itself. In response, Mach said that, contra Newton, the two spheres would move toward one another thanks to the tension in the cord, and if we rotated a very thick, massive ring around a stationary bucket, we would see the water climb the bucket wall. Mach's counter thought experiment undermines our confidence in Newton's. Absolute space explained the phenomena in Newton's thought experiments, but now we're not so sure of the phenomena itself (at least, this is Mach's intent).

Figure 3. Stages in the bucket experiment Figure 4. Two spheres held by a cord in otherwise empty space

Frank Jackson created a much discussed thought experiment that aimed to show that physicalism is false. This is the doctrine that all facts are physical facts. In the thought experiment, Mary is a brilliant scientist who, from birth, is confined to a laboratory with only black and white experiences. She learns all the physical facts about perception there. One day, she leaves the laboratory and experiences colours for the first time; she learns what it's like to experience red. Clearly, says Jackson, she learns something new. Since she already knew all the physical facts, she must have learned something non-physical when she experienced colour. Thus, physicalism must be wrong.

Dennett replied to this thought experiment with one of his own. It begins like Jackson's, but when Mary leaves the lab, she says “Ah, colour perception is just as I thought it would be.” Like Mach, Dennett denies the phenomenon of the original thought experiment. And like Mach, his counter thought experiment is effective in undermining Jackson's in so far as it seems similarly plausible.

To be effective, counter thought experiments needn't be very plausible at all. In a court of law, the jury will convict provided guilt is established "beyond a reasonable doubt." A common defence strategy is to provide an alternative account of the evidence that has just enough plausibility to put the prosecution's case into some measure of doubt. That is sufficient to undermine it. A good counter thought experiment need only do that much to be effective.

Thought experiments can also be constructive. There are many ways a thought experiment could provide positive support for a theory. One of these is to provide a kind of illustration that makes a theory's claims clear and evident. In such cases thought experiments serve as a kind of heuristic aid. A result may already be well established, but the thought experiment can lead to a very satisfying sense of understanding. Newton provided a wonderful example showing how the moon is kept in its orbit in the just same way as an object falls to the earth. He illustrated this by means of a cannon shooting a cannon ball further and further. In the limit, the earth curves away as fast as the ball falls, with the eventual result being that the cannon ball will return to the spot where it was fired, and, if not impeded, will go around again and again. This is what the moon is doing. We could arrive at the same conclusion through calculation. But Newton's thought experiment provides that illusive understanding. It's a wonderful example of the “aha effect.”

Figure 5. “The shot heard around the world”

Einstein's elevator showed that light will bend in a gravitational field; Maxwell's demon showed that entropy could be decreased; Thompson's violinist showed that abortion could be morally permissible even when the fetus has a right to life; Newton's bucket showed that space is a thing in its own right; Parfit's splitting persons showed that survival is a more important notion than identity when considering personhood. I say they “showed” such and such, but, “purport to show” might be better, since some of these thought experiments are quite contentious. The thing they have in common is that they aim to establish something positive. Unlike destructive thought experiments, they are not trying to demolish an existing theory, though they may do that in passing.

Thomas Kuhn's "A Function for Thought Experiments" employs many of the concepts (but not the terminology) of his well-known Structure of Scientific Revolutions . On his view a well-conceived thought experiment can bring on a crisis or at least create an anomaly in the reigning theory and so contribute to paradigm change. Thought experiments can teach us something new about the world, even though we have no new empirical data, by helping us to re-conceptualize the world in a better way. Tamar Gendler has recently developed this view in a number of important respects.

Recent years have seen a sudden growth of interest in thought experiments. The views of Brown (1991) and Norton (1991, 1996) represent the extremes of platonic rationalism and classic empiricism, respectively. Norton claims that any thought experiment is really a (possibly disguised) argument; it starts with premisses grounded in experience and follows deductive or inductive rules of inference in arriving at its conclusion. The picturesque features of any thought experiment which give it an experimental flavour might be psychologically helpful, but are strictly redundant. Thus, says Norton, we never go beyond the empirical premisses in a way to which any empiricist would object. (For criticisms see Bishop 1999; Brown 1991, 2004a, 2004b; Haggqvist 1996; Gendler 1998, 2004; Nersessian 1993; and Sorenson 1992; and for a defense see Norton 1991, 1996, 2004a, and 2004b.)

By contrast, Brown holds that in a few special cases we do go well beyond the old data to acquire a priori knowledge of nature. (See also Koyré 1968.) Galileo showed that all bodies fall at the same speed with a brilliant thought experiment that started by destroying the then reigning Aristotelian account. The latter holds that heavy bodies fall faster than light ones ( H > L ). But consider (Fig. 6), in which a heavy cannon ball ( H ) and light musket ball ( L ) are attached together to form a compound object ( H + L ); the latter must fall faster than the cannon ball alone. Yet the compound object must also fall slower, since the light part will act as a drag on the heavy part. Now we have a contradiction. ( H + L > H and H > H + L ) That's the end of Aristotle's theory. But there is a bonus, since the right account is now obvious: they all fall at the same speed ( H = L = H + L ).

Figure 6. Galileo: “I don't even have to look”

This could be said to be a priori (though still fallible) knowledge of nature, since there are no new data involved, nor is the conclusion derived from old data, nor is it some sort of logical truth. This account of thought experiments can be further developed by linking the a priori epistemology to recent accounts of laws of nature that hold that laws are relations among objectively existing abstract entities. It is thus a rather Platonistic view, not unlike Platonistic accounts of mathematics such as that urged by Gödel. (For details see Brown 1991.)

The two views just sketched might occupy the opposite ends of a spectrum of positions on thought experiments, at least within the philosophy of science. Some of the promising alternative views include those of Sorensen (somewhat in the spirit of Mach) who holds that thought experiments are a "limiting case" of ordinary experiments; they can achieve their aim, he says, without being executed. (Sorensen's book is also valuable for its extensive discussion of thought experiments in a wide range of fields.) Other promising views include those of Gooding (who stresses the similar procedural nature of thought experiments and real experiments), Miscevic and Nersessian (each of whom tie thought experiments to "mental models"), and several of the accounts in Horowitz and Massey 1991. Besides these, a sample of recent excellent discussions includes: Arthur 1999; Gendler 1998, 2000, 2002a, 2004; Haggqvist 1996; Humphreys 1994; McAllister 1996, 2004; and many others. German readers will find the very recent book by Kühne (2005) a very thorough history as well as an interesting discussion of contemporary topics. The literature on thought experiments in the sciences continues to grow rapidly.

Outside of the philosophy of science, philosophers continue to debate the merits of particular thought experiments such as Searle's, Thompson's, Jackson's, and so on. At a more general level there is debate over the usefulness of highly contrived examples. Just how reliable are our intuitions in these cases anyway? The subject of intuition has itself been the topic of recent debate. A small but significant group of philosophers uphold their use while others downplay their reliability and significance. (See DePaul and Ramsey 1998 for a sample of articles on this topic.) The relationship between conceivability and possibility is another topic that has been aired recently and has much to do with thought experiments. (See Gendler and Hawthorne 2002.) The relation between thought experiments and literary fiction is starting to be explored. (See Swirski 2007.)

Thanks are due to Tamar Gendler, from whom I borrowed heavily in constructing the bibliography below. Much more can be found in the bibliographies in Sorenson 1992, Gendler 2000, and Kühne 2005.

• Arthur, R., 1999, "On Thought Experiments as A Priori Science," International Studies in the Philosophy of Science , 13/3: 215-229
• Bishop, M., 1998, "An Epistemological Role for thought Experiments", in N. Shanks (ed.), Idealization IX: Idealization in Contemporary Physics , Amsterdam: Rodopoi, pp. 19-33
• Bishop, M., 1999, "Why Thought Experiments are Not Arguments", Philosophy of Science , 66 : 534-41
• Bokulich, A., 2001, "Rethinking Thought Experiments", Perspectives on Science , 9/3: 285-307
• Brendel, Elke, 2004, "Intuition Pumps and the Proper Use of Thought Experiments", Dialectica , 58/1: 88-108
• Brown, James Robert, 1991, Laboratory of the Mind: Thought Experiments in the Natural Sciences , London: Routledge
• Brown, James Robert, 1993, "Why Empiricism Won't Work." Proceedings of the Philosophy of Science Association , 2: 271-279
• Brown, J.R., 2004a, "Why Thought Experiments Transcend Experience," in C. Hitchcock (ed.), Contemporary Debates in the Philosophy of Science , Malden, MA: Blackwell, pp. 23-43
• Brown, James Robert, 2004b, "Peeking into Plato's Heaven." Philosophy of Science , vol. 71, 1126-1138
• Bunzl, Martin, 1996, "The Logic of Thought Experiments." Synthese , 106/2 (Fall): 227-240
• Buzzoni, Marco, 2004, Esperimento Ed Esperimento Mentale , Milano: FrankoAngeli
• Cargile, James, 1987, "Definitions and Counterexamples." Philosophy , 62: 179-193
• Cohen, M., 2005, Wittgenstein's Beetle and Other Classic Thought Experiments , Oxford: Blackwell
• Cohnitz, Daniel, 2006, Gedankenexperimente in der Philosophie , Paderborn: Verlag GmbH
• Cooper, Rachel, 2005, Metaphilosophy 36:3, 328
• Dancy, Jonathan, 1985, "The Role of Imaginary Cases in Ethics." Pacific Philosophical Quarterly , 66 (January-April): 141-153
• Dennett, D., 1991, Consciousness Explained , New York: Little Brown
• Dennett, D., 2005, Sweet Dreams , Cambridge, MA: MIT Press
• DePaul, M. And W. Ramsey (eds.), 2002, Rethinking Intuition: The Psychology of Intuition & Its Role in Philosophical Inquiry , New York: Rowan and Littlefield
• Duhem, P., 1954, Aim and Structure of Physical Theory , Princeton: Princeton University Press
• Gendler, Tamar Szabo, 1998, "Galileo and the Indispensability of Scientific Thought Experiment." The British Journal for the Philosophy of Science , 49/3 (Sept): 397-424
• Gendler, Tamar Szabo, 2000, Thought Experiment: On the Powers and Limits of Imaginary Cases. NY: Garland Press (now Routledge).
• Gendler, Tamar Szabo, 2002a, "Personal Identity and Thought-Experiments." Philosophical Quarterly , 52/206: 34-54.
• Gendler, Tamar Szabo., 2002b, "Thought Experiment." Encyclopedia of Cognitive Science . NY/London: Nature/Routledge.
• Gendler, Tamar Szabo, 2004, "Thought Experiments Rethought — and Reperceived." Philosophy of Science , 71: 1152-1164.
• Gendler, Tamar Szabo, 2005, "Thought Experiments in Science." Encyclopedia of Philosophy . New York: MacMillan
• Gendler, Tamar Szabo and John Hawthorne, eds., 2002, Conceivability and Possibility. NY/Oxford: Clarendon/Oxford University Press.
• Genz, H., 1999, Gedankenexperimente , Weinheim: Wiley-VCH (in German)
• Gooding, D., 1993, "What is Experimental About Thought Experiments?" in D. Hull, M. Forbes, and K. Okruhlik (eds.) PSA 1992 , vol. 2, East Lansing, MI: Philosophy of Science Association, pp. 280-290
• Gooding, David C., 1992, "The Cognitive Turn, or, Why Do Thought Experiments Work?" In Giere ed., Cognitive Models of Science . Minneapolis: University of Minnesota Press, 1992, 45-76
• Gooding, David C., 1994, "Imaginary Science." British Journal for the Philosophy of Science , 45/4 (December): 1029-1045
• Hacking, I., 1993, "Do Thought Experiments have a Life of Their Own?" in D. Hull, M. Forbes, and K. Okruhlik (eds.) PSA 1992 , vol. 2, East Lansing, MI: Philosophy of Science Association, pp. 302-308
• Haggqvist, S., 1996, Thought Experiments in Philosophy , Stockholm: Almqvist & Wiksell International
• Horowitz, T. and G. Massey (eds.), 1991, Thought Experiments in Science and Philosophy , Savage, MD: Rowman and Littlefield
• Humphries, P., 1994, "Seven Theses on Thought Experiments", in J. Earman et al ., (eds) Philosophical Problems of the Internal and External World , Pittsburgh: University of Pittsburgh Press, pp. 205-227
• Ierodiakonou, K., 2005, "Ancient Thought Experiments: A First Approach", Ancient Philosophy , 25: 125-140
• Irvine, A., 1991, "Thought Experiments in Scientific Reasoning," in Horowitz and Massey 1991, pp. 149-166
• Jackson, F., 1982, "Epiphenomenal Qualia", Philosophical Quarterly , 32: 27-36
• Jackson, M. W., 1992, "The Gedankenexperiment Method of Ethics." The Journal of Value Inquiry , 26: 525-535
• Janis, Allen I., 1991, "Can Thought Experiments Fail?" In Horowitz and Massey 1991, pp. 113-118
• King, Peter, 1991, "Mediaeval Thought-Experiments: The Metamethodology of Mediaeval Science." In Horowitz and Massey 1991, pp. 43-64
• Klassen, S., 2006, "The Science Thought Experiment: How Might it be Used Profitably in the Classroom?", Interchange 37/1: 77-96.
• Koyré, Alexandre, 1968, Metaphysics and Measurement . London: Chapman and Hall.
• Kuhn, T., 1964, "A Function for Thought Experiments", reprinted in T. Kuhn, The Essential Tension , Chicago: University of Chicago Press, 1977, pp. 240-265
• Kühne, U., 2005, Die Methode des Gedankenexperiments , Frankfurt: Suhrkamp
• Kujundzic, Nebojsa, 1992, "How Does the Laboratory of the Mind Work?" Dialogue , 32/3 (Summer): 573-578
• Kujundzic, Nebojsa, 1995, "Thought Experiments: Architecture and Economy of Thought ." The Journal of the British Society for Phenomenology , 26/1 (January): 86-93.
• Laymon, Ronald, 1991, "Thought Experiments of Stevin, Mach and Gouy: Thought Experiments as Ideal Limits and as Semantic Domains." In Horowitz and Massey 1991, pp. 167-192.
• Lennox, James G., 1991, "Darwinian Thought Experiments: A Function for Just-So Stories." In Horowitz and Massey 1991, pp. 223-245
• Lichtenberg, Georg Christoph, 1983, Schriften und Briefe : Sudelbücher, Fragmente, Fabeln, Verse (Erster Band). Ed. Franz H. Mautner. Frankfurt: Insel Verlag
• Mach, E., 1960, The Science of Mechanics , trans. by J. McCormack, sixth edition, LaSalle Illinois: Open Court
• Mach, E., 1976, "On Thought Experiments", in Knowledge and Error , trans. by J. McCormack), Dordrecht: Reidel, pp. 134-147
• Massey, Gerald, 1991, "Backdoor Analyticity." In Horowitz and Massey 1991, pp. 285-296
• McAllister, James, 1996, "The Evidential Significance of Thought Experiments in Science", Studies in History and Philosophy of Science , 27/2: 233-250
• McAllister, James, 2004, "Thought Experiments and the Belief in Phenomena" Proceedings of the 2002 Biennial Meeting of the Philosophy of Science Association, Philosophy of Science , 71: 1164-1175
• McAllister, James, 2005, "The Virtual Laboratory: Thought Experiments in Seventeenth-Century Mechanics", in Helmar Schramm, Ludger Schwarte, and Jan Lazardzig, eds., Collection, Laboratory, Theater: Scenes of Knowledge in the 17th Century . New York: Walter de Gruyter; pp. 35-56
• Miscevic , N., 1992, "Mental Models and Thought Experiments", International Studies in the Philosophy of Science , 6/3: 215-226
• Miscevic, Nenad, 1997, "Categorial and Essentialist Intuitions: A Naturalist Perspective." Acta Analytica 12/19: 21-39
• Nersessian, Nancy, 1992, "How Do Scientists Think? Capturing the Dynamics of Conceptual Change in Science." In R. Giere (ed), Cognitive Models of Science . Minneapolis: University of Minnesota Press, pp. 3-44
• Nersessian, N., 1993, "In the Theoretician's Laboratory: Thought Experimenting as Mental Modeling" in D. Hull, M. Forbes, and K. Okruhlik (eds.) PSA 1992 , vol. 2, East Lansing, MI: Philosophy of Science Association, pp. 291-301
• Norton, J., 1991, "Thought Experiments in Einstein's Work", in Horowitz and Massey 1991, pp. 129-148
• Norton, J., 1996, "Are Thought Experiments Just What You Always Thought?" Canadian Journal of Philosophy , 26: 333-366
• Norton, J., 2004a, "On Thought Experiments: Is There More to the Argument?" Proceedings of the 2002 Biennial Meeting of the Philosophy of Science Association, Philosophy of Science , 71: 1139-1151. [ Preprint available online ].
• Norton, J., 2004b, "Why Thought Experiments Do Not Transcend Empiricism", in Christopher Hitchcock (ed.) Contemporary Debates in the Philosophy of Science. Oxford: Blackwell, pp. 44-66. [ Preprint available online ].
• Parfit, Derek, 1984/1987, Reasons and Persons . Oxford: Clarendon Press
• Rescher, N., 2005, What If?: Thought Experimentation in Philosophy , New Brunswick, NJ: Transaction Publishers
• Schildknecht, Christiane, 1990, Philosophische Masken: Literarische Formen der Philosophie bei Platon, Descartes, Wolff und Lichtenberg. Stuttgart: Metzler
• Sorensen, R., 1992a, Thought Experiments , Oxford: Oxford University Press
• Sorensen, Roy, 1992b, "Thought Experiments and the Epistemology of Laws." Canadian Journal of Philosophy , 22/1 (March): 15-44
• Stinner, A., 1990, Philosophy, Thought Experiments, and Large Context Problems in the Secondary Physics Course. International Journal of Science Education , 12/3: 244-157
• Swirski, Peter, 2007, Of Literature and Knowledge: Explorations in Narrative Thought Experiments, Evolution and Game Theory . London & New York: Routledge
• Thompson, Judith Jarvis, 1971, "A Defense of Abortion." Philosophy and Public Affairs , 1/1 (Fall): 47-66
• Tittle, P., 2005, What If…Collected Thought Experiments in Philosophy , New York: Pearson Longman
• Urbaniec, Jacek, 1988, "In Search of a Philosophical Experiment." Metaphilosophy , 19 (July-August): 294-306
• Wilkes, Kathleen V., 1988, Real People: Personal Identity without Thought Experiments . Oxford: Clarendon Press
• Witt-Hansen, Johannes, 1976, "H.C. Orsted, Immanuel Kant, and the Thought Experiment." Danish Yearbook of Philosophy , 13: 48-65
• Yablo, Stephen, 1993, "Is Conceivability a Guide to Possibility?" Philosophy and Phenomenological Research , 53/1 (March): 1-42
• Goodies , a collection of intriguing questions in the philosophy of science, some about thought experiments, by John Norton (U. Pittsburgh).

[Please contact the author with additional suggestions.]

Descartes, René | -->intuition --> | Leibniz, Gottfried Wilhelm | Mach, Ernst | Platonism: in metaphysics | rationalism vs. empiricism | -->science, philosophy of -->

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Thought Experiments by James Robert Brown , Michael T. Stuart LAST REVIEWED: 29 July 2020 LAST MODIFIED: 29 July 2020 DOI: 10.1093/obo/9780195396577-0143

Thought experiments are performed in the imagination. We set up some situation, we observe what happens, then we try to draw appropriate conclusions. In this way, thought experiments resemble real experiments, except that they are experiments in the mind. The terms “thought experiment,” “imaginary experiment,” and “Gedankenexperiment” are used interchangeably. There is no consensus on a definition, but there is widespread agreement on which are standard examples. It is also widely agreed that they play a central role in a number of fields, especially physics and philosophy. There are several important questions about thought experiments that naturally arise, including what kinds of thought experiments there are, what roles they play, and how, if at all, they work. This last question has been the focus of much of the literature: How can we learn something new about the world just by thinking? Answers range from “We don’t really learn anything new” to “We have some sort of a priori insight into how nature works.” In between there are a great variety of rival alternative accounts. There is still no consensus; debate is wide open on almost every question pertaining to thought experiments.

There has always been some interest in the nature of thought experiments, but it is only in recent years that it has become a popular topic of philosophical interest. Arcangeli 2017 and Stuart, et al. 2018 provide recent overviews of the issues. Brown and Fehige 2019 offers a periodically updated survey of thought experiments and the literature on it. Otherwise, the early works from the current period may be the best place to start, since they provide lots of examples and have tended to set the agenda for subsequent discussion. Horowitz and Massey 1991 is one of the first works stimulating the current interest in thought experiments. Brown 2011 (originally published in 1991) is an early work with many examples. For German readers, Cohnitz 2006 and Kühne 2005 both offer extensive coverage of many topics. Häggqvist 1996 is a critical survey, with an emphasis on modal considerations. Rescher 2005 and Sorensen 1992 both cover a variety of issues and provide many examples.

Arcangeli, Margherita. “Thought Experiments in Model-Based Reasoning.” In Springer Handbook of Model-Based Science . Edited by Lorenzo Magnani and Tommaso Bertolotti, 463–495. Dordrecht, The Netherlands: Springer, 2017.

DOI: 10.1007/978-3-319-30526-4_21

An overview of the recent literature on thought experiments, with a focus on model-based reasoning.

Brown, James Robert. Laboratory of the Mind: Thought Experiments in the Natural Sciences . 2d ed. New York: Routledge, 2011.

DOI: 10.4324/9780203847794

An early work with several standard examples and a taxonomy classifying the different forms that thought experiments take. The author argues for a rationalistic, or Platonistic account of thought experiments, claiming that in some (but not all) we gain a priori access to the abstract realm of laws of nature. Originally published in 1991.

Brown, James Robert, and Yiftach Fehige. “Thought Experiments.” In The Stanford Encyclopedia of Philosophy . Edited by Edward N. Zalta. Stanford, CA: Stanford University, 2019.

A survey of the major issues with a comprehensive bibliography, periodically updated.

Cohnitz, Daniel. Gendankenexperimente in der Philosophie . Paderborn, Germany: Mentis, 2006.

Presents an argument for the usefulness of thought experiments in philosophy. Extensive discussion of different theories of modality to defend thought experiments in philosophy for different purposes, much like conceptual analysis.

Gendler, Tamar S. Thought Experiment: On the Powers and Limits of Imaginary Cases . Abingdon, UK, and New York: Routledge, 2000.

A revised version of Gendler’s PhD thesis, which discusses the role of imagination in thought experiments that use “exceptional cases” to generate new knowledge. Focuses on three case studies: Galileo’s falling bodies, Theseus’s Ship, and Parfit’s fission thought experiment concerning personal identity. Provides four separate and useful bibliographies.

Häggqvist, Sören. Thought Experiments in Philosophy . Stockholm: Almqvist & Wiksell, 1996.

A critical discussion of the early rival accounts of thought experiments. Especially concerned with the relation between thought experiments and modal notions (necessity and possibility).

Horowitz, T., and G. Massey, eds. Thought Experiments in Science and Philosophy . Proceedings of a conference held at the Center for Philosophy at the University of Pittsburgh, 18–20 April 1986. Savage, MD: Rowman & Littlefield, 1991.

Stems from a conference at the University of Pittsburgh in 1986 and contains several excellent and influential articles on a wide range of topics. It is currently out of print, but fortunately, a PDF of the whole book is available online .

Kühne, Ulrich. Die Methode des Gedankenexperiments . Frankfurt: Suhrkamp, 2005.

Comprehensive study of the history of inquiry into thought experiments from Kant to the Brown-Norton debate. Noteworthy are the chapters on Ørsted and Einstein. Argues that Ørsted’s notion of thought experiment is hopelessly confusing and that Einstein, contrary to widespread belief, did not approve of the method of thought experiments.

Rescher, Nicholas. What If? Thought Experimentation in Philosophy . New Brunswick, NJ: Transaction, 2005.

A general and less specialized discussion of thought experiments, includes several historically famous examples. Explores the distinctions between thought experiments and real experiments.

Sorensen, Roy. Thought Experiments . Oxford: Oxford University Press, 1992.

Very wide-ranging. Covers a great many topics in both philosophy and the sciences, and provides a great many examples and deep insights on many issues. One of the author’s central claims is that thought experiments are experiments that merely have not been performed. Develops a theory of the epistemic power of thought experiments in terms of Darwinian evolution.

Stuart, Michael T., Yiftach Fehige, and James R. Brown. “Thought Experiments: State of the Art.” In The Routledge Companion to Thought Experiments . Edited by Michael T. Stuart, Yiftach Fehige, and James R. Brown, 1–28. Abingdon, UK, and New York: Routledge, 2018.

Provides an overview of the literature with examples, and a brief history of the philosophy of thought experiments.

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• Personality

The Power of Thought Experiments

What thought experiments can teach us about ourselves..

Posted June 8, 2022 | Reviewed by Lybi Ma

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Part I of II

Thought experiments are a way to craft free experiences in our mind, designed to answer a question or validate a hypothesis we have about human nature without imposing any demands on us for physical resources, time, or risk-taking .

The greatest scientist of modern time, Einstein, arrived at many of his breakthroughs via thought experiments. At the age of 16, he asked himself, “What if you could ride alongside a beam of light? What would that be like?” It seemed to him that you would perceive a light wave to be stationary since you were traveling at the same speed. He kept deepening his understanding of this concept for the next 10 years until it led to his famous theory of relativity. What’s remarkable is that the experimental proof of some of the propositions of his theory required the building of a particle accelerator that was several miles long and cost billions of dollars. But the actual breakthrough came from a thought experiment in Einstein’s mind. It didn’t even require a pen and napkin.

Just like Einstein used thought experiments to make discoveries about nature, we can use thought experiments to make discoveries about human nature–and about our true nature. Start by formulating a hypothesis, a conjecture about your true nature or human nature that you wish to prove. Design a mental experience, a situation in which you place yourself where this conjecture will be tested. Note the thoughts, feelings, and behaviors that flow freely from the core of your being as you visualize yourself in this situation. Then ask yourself, what understanding does this visualization give me about what values and behaviors are most authentic to me?

Discovering Your True Self

Many people strongly identify with their personality . If you see yourself as an introvert , for instance, you may believe it is natural for you to be reserved in group meetings and get-togethers, not wanting to attract attention to yourself. But what if your true self was something beyond your personality?

Imagine a moment where you are walking in a bustling city with your 10-year-old son or nephew. He breaks free from your grasp and turns to run across the street. There’s traffic all around.

Will you in that moment say to yourself, “I can’t raise my voice! I am an introvert. I don’t feel comfortable shouting in public and attracting all kinds of attention.” Or will you shout at the top of your voice? “Jamie! Stop!” Would you feel more true in that moment by acting like the introvert you see yourself to be, or by acting just the opposite?

A simple thought experiment ends up revealing a powerful truth: We feel more true to ourselves when our behavior is motivated by what we deeply care about, even if this behavior is the opposite of our personality. When it comes to being authentic, purpose triumphs over personality.

In years past, when I met people who were highly successful executives in a certain industry, or highly successful academics, I would do a thought experiment, “What if this is the career path I pursued, and this is where I ended up. Would I feel deeply fulfilled?” Most of the time, the answer I received from within was, “No.” While I admired them and looked up at them, and while others might seek to emulate them, this thought experiment showed me that their path was not going to be my path; my true self was seeking something else, even if I did not know what it was at that time.

The people I was most drawn to visualizing as my end-state were typically either creative people–movie directors, authors–or spiritual truth-seekers. A thought experiment of visualizing who I would need to be in the future for me to feel fulfilled helped me not get entrapped in career paths that weren’t true to me.

Developing Empathy

In studying Mother Teresa’s life, I initially found myself deeply inspired by her commitment to serving the poorest among the poor. But I also learned that she had consorted at times with people of disrepute, such as a banker convicted of a crime and a dictator. I wondered why she was not more discriminating in whom she engaged with for her cause. Then I constructed a thought experiment.

I visualized that I was traveling in a remote country with a loved one. Our car meets with an accident, and my loved one is seriously injured. Her life is in danger as she bleeds by the side of the road. There is no help in sight. One car passes by on the road, and despite our entreaties, it does not stop. Nor do the next 10. The twelfth car I try to wave down stops to help us. I am overwhelmed with relief. Then I suddenly notice that the driver of the vehicle is a corrupt dictator I have long been critical of. What would I do in that moment?

Would I ask him to leave, because I have judged him to be a bad person? Or would I jump at his offer to help, thank him for doing so, and focus on getting my loved one to the nearest hospital? I realized that this was probably what Mother Teresa had experienced. Her loved ones were the street people that most of the world had abandoned like those 11 cars that I had imagined passing my dying loved one. Whoever offered help, she received it with gratitude and without judgment. Her business was not to investigate their lives or support their agenda; it was to attract love, care, and support for the people on the sidewalks that most of us had chosen to pass by. This thought experiment taught me the importance of cultivating empathy. We cannot judge someone’s character based on a behavior we see without first trying to see the world through their eyes. We need to understand their motivations and context before critiquing their behavior, and thought experiments can help us do so.

Hill, P. L., Cheung, F., Kubel, A., & Burrow, A. L. Life engagement is associated with higher GDP among societies. (2019). Journal of Research in Personality, 78, 210-214.

Hitendra Wadhwa, Ph.D. , is Professor of Practice at Columbia Business School, the Founder of the Mentora Institute, and the author of Inner Mastery, Outer Impact.

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Top 10 Most Famous Thought Experiments

Thought experiments are mental concepts or hypotheses, often resembling riddles, which are used by philosophers and scientists as simple ways of illuminating what are usually very dense ideas. Most often, they’re used in more abstract fields like philosophy and theoretical physics, where physical experiments aren’t possible. They serve as some hearty food for thought, but given their complex subject matter, it’s not unusual for even the thought experiment itself to be nearly incomprehensible. With this in mind, here are ten of the most famous thought experiments , along with explanations of the philosophical, scientific, and ethical ideas they work to explain:

10. The Trolley Problem

One of the most well known thought experiments in the field of ethics is the “Trolley Problem,” which goes something like this: a madman has tied five innocent people to a trolley track. An out of control trolley car is careening toward them, and is moments away from running them over. Luckily, you can pull a lever and divert the trolley to another track. The only problem is that the madman has also tied a single person to that track. Considering the circumstances, should you pull the lever?

What it Means:

The trolley problem was first proposed by the philosopher Philippa Foot as a means of critiquing the major theories in ethical philosophy, in particular utilitarianism, the system which proposes that the most moral decision is always the one that provides “the greatest good for the greatest number.” From a utilitarian point of view, the obvious choice is to pull the lever, saving five and only killing one. But critics of this theory would state that in pulling the lever you become complicit in what is clearly an immoral act—you are now partially responsible for the death of the lone person on the other track. Others, meanwhile, argue that your mere presence in the situation demands that you act, and that to do nothing would be equally immoral. In short, there is no wholly moral action, and this is the point. Many philosophers have used the trolley problem as an example of the ways that real world situations often force individuals to compromise their own moral codes, and that there are times when there is no totally moral course of action.

9. The Cow in the Field

One of the major thought experiments in epistemology (the field of philosophy that deals with knowledge) is what is known as “The Cow in the Field.” It concerns a farmer who is worried his prize cow has wandered off. When the milkman comes to the farm, he tells the farmer not to worry, because he’s seen that the cow is in a nearby field. Though he’s nearly sure the man is right, the farmer takes a look for himself, sees the familiar black and white shape of his cow, and is satisfied that he knows the cow is there. Later on, the milkman drops by the field to double-check. The cow is indeed there, but it’s hidden in a grove of trees. There is also a large sheet of black and white paper caught in a tree, and it is obvious that the farmer mistook it for his cow. The question, then: even though the cow was in the field, was the farmer correct when he said he knew it was there?

The Cow in the Field was first used by Edmund Gettier as a criticism of the popular definition of knowledge as “justified true belief”—that is, that something becomes knowledge when a person believes it; it is factually true; and they have a verifiable justification for their belief. In the experiment, the farmer’s belief that the cow was there was justified by the testimony of the milkman and his own verification of a black and white object sitting in the field. It also happened to be true, as the milkman later confirmed. But despite all this, the farmer did not truly know the cow was there, because his reasoning for believing it turned out to be based on false premises. Gettier used this experiment, along with a few other examples, as proof of his argument that the definition of knowledge as justified true belief needed to be amended.

8. The Ticking Time Bomb

If you’ve paid any attention to political discourse over the past few years—or ever seen an action movie, for that matter—then you are no doubt familiar with the “ticking time bomb” thought experiment. It asks you to imagine that a bomb or other weapon of mass destruction is hidden in your city, and the timer on it will soon strike zero. You have in your custody a man with knowledge of where the device is planted. Do you resort to torture in order to get him to give up the information?

Like the trolley problem, the ticking time bomb scenario is an ethical problem that forces one to choose between two morally questionable acts. It is most often employed as a counter argument to those who say the use of torture is inexcusable under any circumstances. It’s also used as an example of the way laws—like those the U.S. has against torturing prisoners—will always be set aside given extreme circumstances. Thanks to its fictionalized use in television shows like 24 , along with its constant position in political debates , the ticking time bomb scenario has become one of the most frequently repeated thought experiments. An even more extreme take on the problem was presented in a British news article earlier this year. That version proposes that the terrorist in question won’t respond to torture, and asks if one would be willing to resort to torturing the man’s wife and children as a means of extracting the information from him.

7. Einstein’s Light Beam

It’s a little known fact that Albert Einstein’s famous work on special relativity was spurred by a thought experiment he conducted when he was only 16 years old. In his book Autobiographical Notes , Einstein recalls how he once daydreamed about chasing a beam of light as it traveled through space. He reasoned that if he were able to move next to it at the speed of light, he should be able to observe the light frozen in space as “an electromagnetic field at rest though spatially oscillating.” For Einstein , this thought experiment proved that for his imaginary observer “everything would have to happen according to the same laws as for an observer who, relative to the Earth, was at rest.”

In truth, no one really knows for sure. Scientists have long debated how this deceivingly simple thought experiment helped Einstein make the massive theoretical leap required to arrive at special relativity theory. At the time, the ideas in the experiment contradicted the now-debunked belief in the “aether,” an invisible field through which light was believed to travel. It would be years before he could prove he was right, but this thought experiment was somehow the “germ,” as he called it, for Einstein’s theory of special relativity, one of the ideas that first established him as a towering figure in theoretical physics.

6. The Ship of Theseus

One of the oldest of all thought experiments is the paradox known as the Ship of Theseus, which originated in the writings of Plutarch. It describes a ship that remained seaworthy for hundreds of years thanks to constant repairs and replacement parts. As soon as one plank became old and rotted, it would be replaced, and so on until every working part of the ship was no longer original to it. The question is whether this end product is still the same Ship of Theseus, or something completely new and different. If it’s not, at what point did it stop being the same ship? The Philosopher Thomas Hobbes would later take the problem even further: if one were to take all the old parts removed from the Ship of Theseus and build a new ship from them, then which of the two vessels is the real Ship of Theseus?

For philosophers, the story of the Ship of Theseus is used as a means of exploring the nature of identity, specifically the question of whether objects are more than just the sum of their parts. A more modern example would be a band that had evolved over the years to the point that few or no original members remained in the lineup. This notion is also applicable to everything from businesses, which might retain the same name despite mergers and changes in leadership, to the human body, which is constantly regenerating and rebuilding itself. At its heart, the experiment forces one to question the commonly held idea that identity is solely contained in physical objects and phenomena .

5. Galileo’s Gravity Experiment

One of the earliest thought experiments originated with the physicist and astronomer Galileo . In order to refute Aristotle’s claim that the speed of a falling object is dictated by its mass, Galileo devised a simple mental example: According to Aristotelian logic, if a light object and a heavy object were tied together and dropped off a tower, then the heavier object would fall faster, and the rope between the two would become taut. This would allow the lighter object to create drag and slow the heavy one down. But Galileo reasoned that once this occurs, the weight of the two objects together should be heavier than the weight of either one by itself, therefore making the system as a whole fall faster. That this is a contradiction proved that Aristotle’s hypothesis was wrong.

One of the most famous stories about Galileo is that he once dropped two metal balls off the Leaning Tower of Pisa to prove that heavier objects do not fall faster than lighter ones. In actuality, this story is probably just a legend; instead, it was this elegant thought experiment that helped prove a very important theory about gravity: no matter their mass, all objects fall at the same rate of speed.

4. Monkeys and Typewriters

Another thought experiment that gets a lot of play in popular culture is what is known as the “infinite monkey theorem.” Also known as the “monkeys and typewriters” experiment, the theorem states that if an infinite number of monkeys were allowed to randomly hit keys on an infinite number of typewriters for an infinite amount of time, then at some point they would “almost surely” produce the complete works of Shakespeare. The monkeys and typewriters idea was popularized in the early 20 th century by the French mathematician Emile Borel, but its basic idea—that infinite agents and infinite time will randomly produce anything and everything—dates back to Aristotle.

Simply put, the “ monkeys and typewriters” theorem is one of the best ways to illustrate the nature of infinity. The human mind has a difficult time imagining a universe with no end or time that will never cease, and the infinite monkeys help to illustrate the sheer breadth of possibilities these concepts create. The idea that a monkey could write Hamlet by accident seems counterintuitive, but in fact it is mathematically provable when one considers the probabilities. The theorem itself is impossible to recreate in the real world, but that hasn’t stopped some from trying: In 2003, science students at a zoo in the U.K. “tested” the infinite monkey theorem when they put a computer and a keyboard in a primate enclosure. Unfortunately, the monkeys never got around to composing any sonnets. According to researchers, all they managed to produce was five pages consisting almost entirely of the letter “s.”

3. The Chinese Room

The Chinese Room is a famous thought experiment first proposed in the early 1980s by John Searle, a prominent American philosopher. The experiment asks you to imagine that an English speaking man has been placed in a room that is entirely sealed, save for a small mail slot in the chamber door. He has with him a hard copy in English of a computer program that translates the Chinese language. He also has plenty of spare scratch paper, pencils, and file cabinets. Pieces of paper containing Chinese characters are then slipped through the slot in the door. According to Searle, the man should be able to use his book to translate them and then send back his own response in Chinese. Although he doesn’t speak a word of the language, Searle argues that through this process the man in the room could convince anyone on the outside that he was a fluent speaker of Chinese.

Searle conceived the Chinese Room thought experiment in order to refute the argument that computers and other artificial intelligences could actually think and understand. The man in the room does not speak Chinese; he can’t think in the language . But because he has certain tools at his disposal, he would be able convince even a native speaker that he was fluent in it. According to Searle, computers do the same thing. They don’t ever truly understand the information they’re given, but they can run a program, access information, and give a clear impression of human intelligence.

2. Schrodinger’s Cat

Schrödinger’s Cat is a paradox relating to quantum mechanics that was first proposed by the physicist Erwin Schrödinger. It concerns a cat that is sealed inside a box for one hour along with a radioactive element and a vial of deadly poison. There is a 50/50 chance that the radioactive element will decay over the course of the hour. If it does, then a hammer connected to a Geiger counter will trigger, break the vial, release the poison, and kill the cat. Since there is an equal chance that this will or will not happen, Schrödinger argued that before the box is opened the cat is simultaneously both alive and dead.

In short, the point of the experiment is that because there is no one around to witness what had occurred, the cat existed in all of its possible states (in this case either alive or dead) simultaneously. This notion is similar to the old “if a tree falls in the woods and there’s no one there to hear it, does it make a sound?” riddle. Schrödinger originally conceived of his theoretical cat in response to an article that discussed the nature of quantum superpositions, a theory that defines all the possible states in which an object can exist. Schrödinger’s Cat also helped to illustrate just how weird the rules of quantum mechanics really were. The thought experiment is notorious for its complexity, which has encouraged a wide variety of interpretations. One of the most bizarre is the “many worlds” hypothesis, which states that the cat is both alive and dead, and that both cats exist in different universes that will never overlap with one another.

1. Brain in a Vat

There has been no more influential thought experiment than the so-called “brain in a vat” hypothesis, which has permeated everything from cognitive science and philosophy to popular culture. The experiment asks you to imagine a mad scientist has taken your brain from your body and placed it in a vat of some kind of life sustaining fluid. Electrodes have been connected to your brain, and these are connected to a computer that generates images and sensations. Since all your information about the world is filtered through the brain, this computer would have the ability to simulate your everyday experience. If this were indeed possible, how could you ever truly prove that the world around you was real, and not just a simulation generated by a computer?

If you’re thinking this all sounds a bit like The Matrix , you’re right. That film, along with several other sci-fi stories and movies, was heavily influenced by the brain in a vat thought experiment. At its heart, the exercise asks you to question the nature of experience, and to consider what it really means to be human. The idea for the experiment, which was popularized by Hilary Putnam, dates all the way back to the 17 th century philosopher Rene Descartes. In his Meditations on the First Philosophy, Descartes questioned whether he could ever truly prove that all his sensations were really his own, and not just an illusion caused by an “evil daemon.” Descartes accounted for this problem with his classic maxim “cogito ergo sum” (“I think therefore I am”). Unfortunately, the brain in a vat experiment complicates this argument, too, since a brain connected to electrodes could still think. The brain in a vat experiment has been widely discussed among philosophers, and many objections have been raised over its premise, but there is still no good rebuttal to its central question: how do you ever truly know what is real ?

68 Comments

I remember the Chinese room! I Think there was a riddle after that. I saw a similar riddle too that I wanted to share here if that was okay: 3 Gods Riddle

If you liked the one about Schrodinger’s Cat, checkout Quantum Suicide: https://thoughtexperiments.net/quantum-suicide/

Long ago (&FA) Star Trek has an episode, “The Squire of Gothos” https://en.wikipedia.org/wiki/The_Squire_of_Gothos Who’s Toys are we?

I have an axe which once belonged to Abraham Lincoln. Since then, it’s had six new heads and six new handles.

Trolley Problem Five innocent lives versus one innocent life seems like an easy decision to make when looking at it from a distance; everyone thinks that saving five people is better than saving one person and to an extent I agree. But the actual person pulling the lever knows nothing about each person, yes they are all innocent but how can that stranger pull a lever and put a value on a life during such a short and stressful situation. That one person could have done more with his life than the five people did together. Or the one person could have been getting his life back together and the five people could have been valued members of their community, involved in everything with a family and a full job. When looking at someone else’s life from the outside no one really knows what they live with and go through in their daily life. It is very hard to judge a person and put a value on their life when they are strangers. As the background information tells us that in a utopian world saving five lives is better than saving one. I think most people would agree with this. After they are saved they could go on and better their lives or change something that they have wanted to. Also in most people’s heads five of almost anything is more valuable than one. Although most people have morals and I’m sure one of them for everyone is a person should not kill another person, being put in that situation a stranger is forced to decide between five people and one person. If they don’t act they become a bystander and they will know that with them doing nothing they watched five people die and one person walk away instead of the other way around. With not knowing anything about anyone I think the stranger should pull the lever, as horrible as it is to put a value on a life saving five people sounds better than saving one.

That’s a great list; the brain in the vat experiment is particularly interesting but I’d love to try out a virtual reality. I’ve heard people saying that the The Matrix ripped off the idea but The Matrix is actually based on Simulacra and Simulacrum, a book that deals with hyperreality, written by Jean Baudrillard.

So which is it? You make two directly opposing statements:

“The probability of each keystroke is completely independant of everyother keystroke. ”

“the probability of getting tails twice in a row when fliping a coin is .25, right? But if you flip the coin and get tails once, the probability rises to .50, because the first variable has become a certainty (1.0).”

Both statements cannot be true, and indeed are not true. The first is correct, the second is the “Gambler’s Fallacy”. Prior coin tosses have zero influence on future coin tosses, just like prior keystrokes have zero influence on future keystrokes.

Keystrokes and coins are no different from each other. Each act of flipping a coin is completely independent of every other act of flipping a coin. The odds do not change just because a particular outcome already happened, or did not happen.

English can’t be your first language and/or you must have failed your one statistics course. The two statements do not counter each other at all. I’ll explain the second statement like I would to my 5 year old niece. The probability of a heads or tails is 1/2 or .5. You with me? One result of two possible outcomes. The probability of getting tails twice in a row is 1/4 or .25. One result of 4 possible outcomes (HH, HT, TH, TT). After flipping the coin once and getting a tails it limits the outcome from the first set to either TH or TT so there is now a 50% chance of flipping TT. You have a little information but no true understanding of anything and all your comments make me weep for the education system.

Maybe there is only 1 original thinker amongst us… the rest are just creations of their mind.

How can #2 be called a Paradox? Its often described as a paradox. But I don’t think its really thought of one at all by physicists. The apparent paradoxical part is the cat is both alive & dead, but what about single atoms & quantum states? They are in superposition & ill defined just like the cat. How can it even be described a paradox if the measurement problem is still unresolved? And than adding decoherence? The experiment only deals with the copenhagen interpretation & argues that it is absurd especially when scaled up to macro scale.

well ive just seen this for the first time in science and ive got to say its very interesting. the theories created arguments between me and friends =) lol any will defo be back to look in more depth

Because the state of the atom determines if the cat is alive or dead and because quantum physics proposes that the atom is in the state in which the cat is both alive and dead, it means that quantum physics do not translate into the real world. That’s why people are trying to find a Universal Theory (such as the proposed String Theory) because quantum mechanics and gravity (i.e. small math, large math) do not agree with each other, yet they are both correct in their calculations.

Well, I could explain it to you (it’s actually not hard to understand at all), but I seem to have used up all my “time to waste” (as you call it) on showing why Number 4 is not correct for any conceivable universe… 🙂 Sorry!

( But here’s a hint: The Schrodinger’s cat paradox is based on observation, just as the entire universe is based on observation. And it has nothing at all to do with anyone observing the cat. The experiment, as laid out here, is not stated correctly as a demonstration of quantum mechanics: If it were, the cat actually would be both alive and dead at the same time, in exactly the same manner as particles on the quantum scale being in multiple indeterminate states at once, until observed. )

I really don’t get Schrodinger’s Cat….. can someone dumb it down further to the level of a 10 year old for me? haha… i’ve tried reading up on quantum mechanics, but somehow my brain refuses to absorb any of it. what exactly is Schrodinger arguing against and what’s his point when actually applied in quantum mechanics?

“because there is no one around to witness what had occurred, the cat existed in all of its possible states”. this particular line doesn’t make sense to me. it’s just dead OR alive so how can anyone say that the cat existed in all of its possible states just because no one knows what happened to it? is this based on an external observer’s viewpoint?

It can. It really depends on how you want to interpret it. There’s a few ways. The one you’re thinking of is one of them.

http://en.wikipedia.org/wiki/Schr%C3%B6dinger's_cat#Interpretations_of_the_experiment

The answer given for #4 is, of course, not correct: The monkeys never can type the entire works of Shakespeare, nor any other similar volume of text, in any realistic amount of time. The reason is simple: each time a monkey starts getting a string of text right, it is far more probable that he will get next keystroke wrong than right, thus invalidating the entire string. The longer the string gets, the more improbable it becomes that the next letter will be correct. For short sequences (just a few letters) the monkey might get it right by shear chance, but for anything longer than a half dozen words, it just won’t happen. Not even in the entire age of the universe.

Think of it this way: Assume that the monkey has a keyboard that can produce 26 uppercase letters, 26 lowercase letters, and a half a dozen punctuation marks (space, full stop, comma, quotes, exclamation mark, and question mark.) That makes 60 characters. The monkey starts typing at random, and produces a “W”, which happens to be the first letter of the first line in Macbeth: (“When shall we three meet again”). Let’s say “the force is with him”, and on the next few keystrokes he hits an “h” an “e” an “n” and a space. Great! Now he has entire first word right! The problem is, on every single “next keystroke”, there are 59 ways the monkey can be wrong, and only one way he can be right. If he does not hit an “s” then everything he has typed so far is useless, the entire text must be scrapped, and he has to start again.

He is 59 times more likely to hit the WRONG key than the RIGHT one, at random. He only has a 1 in 60 chance that the next letter will be correct.

But this isn’t just a matter of adding up the letters and multiplying by 60: this is an exponential problem. The chances that he will get two letters in a row correct are one in 60×60, which is also written 60^2, which works out to 3600. So he’d need to type 3600 random keystrokes to be stand a good chance of producing the first two letters (“Wh”)

The chances of getting three letters right are 1 in 60^3 (60x60x60), which is one in 216,000. For 4 letters is one in 12,960,000. To get “When” right, he’d need to type about 13 million keystrokes! Maybe you are starting to get the picture…

Mathematically, the chances that he’ll get “n” letters correct are 60^n (“60 to the power n”, which is the mathematical way of saying “there are 60 ways of getting each of “n” letters.).

The entire first line of Macbeth (“When shall we three meet again?”) has 31 characters, so the chances of the monkey getting it right are 60^31, which is roughly one in 1,326,443,500,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000. Wow!

Let’s say this monkey types really fast, never sleeps, eats or takes vacations, and can tap out 10 characters per second. So roughly every 3 seconds he will produce a line of text that we can compare against that first line from Macbeth. How long will it take the monkey to produce that line? One sample every 3 seconds is 20 samples per minute, which is 1200 per hour, 28,800 per day, and 10,519,200 per year. Ten million samples per year! Not bad…. At that rate, it will only take the monkey a bit more than a year to type the single word “When” correctly, and about 126,097,376,067,039,332,591,704,312,114,990,000,000,000,000,000 years to get the entire first line of Macbeth correct! Well, it turns out that, so far, the entire universe has only aged about 15,000,000,000 years (give or take a billion), so that poor hard-working monkey is going to need a bit more time…

So we add more monkeys! Let’s be generous and put not just ONE monkey to work, but a BILLION monkeys to work. Cool! Between them, it will only take them about 1,260,973,760,670,393,325,917,043,121,149,900,000,000 years! That’s MUCH better, isn’t it?

Hmm, so it seems a billion monkeys working for the entire age of the universe isn’t enough. So let’s get REAL generous, and say that there will be one billion monkeys on each of one billion planets in each of one billion galaxies… how does that work out?

Turns out, we are now down to just 1,260,973,760,670,393,325,917 years! Or roughly 840,649,173,780 TIMES the age of the universe.

In other words, a billion billion billion monkeys, typing for the entire age of the universe, (ever since the Big Bang and right up to now), multiplied eight hundred and forty billion times over, stand a roughly even chance of producing ONLY THE FIRST LINE OF MACBETH correctly!!!! Just 31 characters.

Maybe now you get to see why this is such an incredibly improbable feat! And that’s just for a very simple phrase: “When shall we three meet again?”.

But Shakespeare wrote quite a bit more than just 31 characters. There are roughly 130,000 characters in “Macbeth”, so instead of the problem being just 60^31 it is actually 60^130,000 for just that one play (Macbeth). But Shakespeare wrote a total of 38 plays, 154 sonnets, two rather long poems, and several shorter poems. ….

Sorry, but that ain’t gonna happen. Not even if you could miniaturize the monkeys, and speed them up a thousand times. In fact, not even if you could get every single atom in the entire known universe typing out text at the rate of millions of characters per second! Even then, you STILL could not get the job done in any realistic amount of time. Not even in an incredibly unrealistic amount of time!

(And all of this is without even considering who is going to CHECK what the monkeys typed, compare it against the works of Shakespeare, and see if they got it right or not…)

That’s the difference between theory and reality. Yeah, with an “infinite” amount of time and an “infinite” number of monkeys you could do it, in theory, but NOT with any realistic scenario of time, monkeys, typewriters or text.

So the claim the correct answer is that all those monkeys and all those typewriters don’t stand even the vaguest chance of producing even the tiniest fraction of the works of Shakespeare. #4 is wrong.

Thank you for one of the most compete replies I have seen on Toptenz.net. Great comment.

wow… u obviously have a lot of time to waste.

the beauty of the concept of “infinity” is that compared to infinity 1,326,443,500,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 raised to itself is almost just like a speck of dust in this… errr…. dusty universe…. in short, unimaginable. who knows, with all those monkeys one may have been bitten by a radioactive clone of shakespeare that has given that said monkey the super power to not only remember all his works verbatim but also type it using a typewriter (heck, if i can think it then it’s probably one of the infinite possibilities, right?)……… “Realistic” has been thrown out of the window when the term “infinite” was used. u don’t need crazy mathematical skills to get the point of this thought experiment, u just need your imagination! =D

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don’t mind me…. i’m just bored.

For waaaaaaaaaayyy too many of these comments, people are not grasping a) the point of a thought experiment in the first place (it’s not literal) and b) the idea or concept behind the experiment! Infinity is a concept, a never ending number. The authors for instance never claimed that we should actually rely on monkeys to type Shakespeare. In the scenario of an infinite number of monkeys with an infinite number of typewriters, every keystroke that goes on has an infinite number of chances at being the correct keystroke, so some of the monkeys get it right one letter at a time. The odds of the next keystroke being correct as well are incredibly slim, but that is where the element of infinite time factors in. Given enough time, these monkeys WILL type every possible combination of characters imaginable, and within those possible combinations will be every written work known to man. The thought experiment never claimed that this would happen in “a reasonable amount of time.” Maybe the give away was the “infinite amount of time” factor. Regardless, please don’t waste your time attempting to poke meaningless holes in a THEOREM by which you are simply spreading your own ignorance of a concept rather than learning for yourself.

When people have to resort to insults and straw-man arguments in their vain attempt to defend an indefensible position, all that it shows is that they don’t actually have any basis for their superfluous wafflings and splutterings at all! 🙂

The point, of course, is that infinite time changes nothing: with every single keystroke, it becomes statistically more and more UNLIKELY that the monkeys can ever type the entire works of Shakespeare! I did, of course, point this out in the very first paragraph of my original post:

“The reason is simple: each time a monkey starts getting a string of text right, it is far more probable that he will get next keystroke wrong than right, thus invalidating the entire string. The longer the string gets, the more improbable it becomes that the next letter will be correct.”

Got that last part? “The longer the string gets, the more improbable it becomes that the next letter will be correct.” In other words, as time goes on toward infinity, then chance of getting it WRONG increase, not the chances of getting it RIGHT.

In other words (for those who seem to have a problem understanding basic high-school statistics, and simple logic), each time that a monkey types a letter, the chances that he will get the next letter WRONG increase EXPONENTIALLY, while the chances he will get it RIGHT only increase LINEARLY. It doesn’t take very long before the probability of getting it wrong approached infinity… And each time a monkey strikes yet another key, the probability that he will fail at the task get even closer to 1, while his chances at succeeding get closer to zero.

On the very first keystroke his chances his chances of getting that one right are very high, but on the second keystroke, the changes drop dramatically. By the twentieth keystroke, his chances are so close to zero as to be not worth mentioning, and as time progresses, his chances get every closer to zero, and not only that, they ACCELERATE towards zero.

The belief to the contrary is, of course, closely related to the gambler’s fallacy: The gambler believes that a string of unbroken losses means that his chances of winning on the next bet are improving, when in fact they are not. The inverses case for the monkeys is similar (although infinitely more negative) : A string of unbroken “wins” (hitting the correct letter) does not increase the chances of success, and in fact increases the chances of FAILURE.

And no, the fact of extending the experiment for an infinite amount of time does NOT make it more probable that the monkeys will eventually succeed: In reality, it makes it infinitely more likely that they will continue to fail, eternally. Inability to see this rather obvious implication is a clear indication of your basic misunderstanding of statistics and the concept of infinity. Despite what intuition tells you, actual reality is somewhat different. No, that’s not a personal insult: it is simply a statement of fact.

So, if you can’t grasp the simple basics of statistical analysis, then maybe you shouldn’t post unfounded opinions on the internet: That way, you could avoid embarrassing yourself further.

The mere fact that you attempted to embarrass ME into not refuting your childish position is a glaringly obvious indicator of your fear of being refuted. Which, I believe, I have accomplished rather successfully anyway. (And which, of course, is your cue to spew forth yet another unfounded, infantile response of meaningless drivel.)

The objective does become more improbable however the probability never becomes zero (impossible). Therefore given infiniate time,a period without end,it will eventually occur.

Don’t forget the infinite monkeys, infinite typewriters, and infinite time. I think you’re trapped into seeing a finite amount of monkeys. If that were the case, then the chances would be decreasing as such. But the probability of recreating such Shakespearean works given infinite monkeys, and infinite time…with the use of infinite keyboards surely must be 1. The probability of each monkey getting it is so small that it would approach zero, but the probability of one of them getting it eventually would approach 1.

Except that there is also an infinite number of possibilities that do not contain the works of Shakespeare. Thus making it possible that even with infinity and an infinite number of monkeys, they may never type the complete works of Shakespeare.

The probability of each keystroke is completely independant of everyother keystroke. The overall probability of the monkeys writing all of billy shakes’ works is inconcievably low before said monkey begins, but becomes exponentially greater each time he selects the correct key. For example, the probability of getting tails twice in a row when fliping a coin is .25, right? But if you flip the coin and get tails once, the probability rises to .50, because the first variable has become a certainty (1.0). And beside that, assuming assumingall the mokeys are working at once, one failure wastes little time or energy, because there are infinite other monkeys typing away, at least a few of them are already killing off there third or fourth character by now. Remember, nobody ever said the circumstances were possible, but given such generous circumstances it is almost certain that the works will come out in a few years.

“In other words (for those who seem to have a problem understanding basic high-school statistics, and simple logic), each time that a monkey types a letter, the chances that he will get the next letter WRONG increase EXPONENTIALLY, while the chances he will get it RIGHT only increase LINEARLY. It doesn’t take very long before the probability of getting it wrong approached infinity… And each time a monkey strikes yet another key, the probability that he will fail at the task get even closer to 1, while his chances at succeeding get closer to zero.”

Those two statistics don’t exist at the same time. They can easily even be considered the same statistic based purely on respect to temporal perspective. Each additional character required causes the initial probability of failure to approach infinity. Each additional character gotten correct, from that point on, literally decreases the amount of required characters for that instance, causing the probability of success to approach 100%. Each incorrect character merely resets the counter. The statistic you keep bringing up is more akin to a starting line and can’t change once monkeys are started typing.

What’s more damning, however, is that that “exponentially increasing” statistic is rooted in the single finite variable in the entire experiment: the length of The Complete Works of Shakespeare.

Because the length of The Complete Works of Shakespeare is finite, it will always be infinitely less than infinity. This is the crux that your argument neglects.

No matter how infinitesimally small the probability of a monkey producing The Complete Works of Shakespeare is, with infinite monkeys and infinite typewriters, you’re directly comparing a finite number to infinity.

You don’t even need infinite time. All possible occurrences are happening simultaneously with the monkeys alone and the time it takes to produce The Complete Works of Shakespeare is literally the shortest time possible for a monkey to type it. There will be an infinite amount of failures and an infinite amount of successes because everything that could ever happen will happen at once an infinite amount of times. All your calculations are ultimately meaningless because infinity breaks math. Once it’s introduced, there’s really only two numbers: infinity and not-infinity. This is why calculus is so convoluted; it bends over backwards to avoid this.

I think Chileman2020 has not in the least grasp of the word infinty. When we are talking of infinte universe, anything having probability of more than zero will recur not 1, 2 but infinite times.so even if probality of monkeys typing the series is 1X10^-100000…… it will happen infinite times.There will also be infinite harry potter collections , infinite oxford dictionaries as well as infinite times aaaaa…./bbbbb…./cccc…. etc.

There are an infinite number of numbers on the decimal line between 1 and 2, but none of them are other whole numbers. Just because you have an infinite series does NOT mean you can count on everything being contained within that series. There are still probabilities in infinite series, especially since we cannot directly observe and quantify the things within that series. That is why it can be said that in an infinite series things may be more or less likely to occur. For example, in this mind experiment it is far more likely that infinite monkeys will poop, play, mate, and fight each other than they are to make intentional keystrokes. The point of the thought experiment is to say that a RANDOM element has the ability to produce non-random things (such as all of Shakespeare). The problem is, random elements cannot be quantified.

If you put a truly random element in a scenario (monkeys are actually pretty predictable) there is NO possible world (a way of saying that no matter how you couch the mind experiment it won’t work) where you can say for certain that the truly random element will produce anything. You cannot even say that the random element will produce nothing (since it is random). Every “set” is possible to the random element but NO set can be proven actual even in an infinite space, with infinite elements, infinite time, and infinite materials. And don’t try to argue that by proving that no set can be proven I’ve proved that the set “no set can be proven” is proved. That is a logical absurdity and to argue it is to cut your own legs off.

I’m not sure my mind is working properly after those last couple of sentences I just read.

That is proof your mind is working properly. 😉

the movie inception deals with number 1 and 2! go see it

Why am I myself, rather than someone else?

This is a very basic question about life, but it may also strike someone as nonsensical. What would it mean for someone else to be me? Or, what would it mean for me to be someone else?

These questions, which are not easily addressed empirically, can be dealt with by way of thought experiments.

First, I can imagine someone else being me if a duplicate were to be made of my body, with all my features, memories, habits, etc., and then if I were to be replaced by it. This was essentially the plot of the science fiction movie The Invasion of the Body Snatchers (1956, 1978), although the duplicates in those cases were not precise copies of the replaced individuals — they were actually alien beings that were duplicates to all external appearances, but not internally. However, it is not hard to imagine true duplicates being made, especially with the kind of technology imagined for the transporter machines in the Star Trek television series. The 6th Day, a recent Arnold Schwarzenegger movie (2000), was about just such complete duplicates. Nature itself produces duplicates, but only in the very first stages of life: Identical twins are genetically the same, but their experiences and memories begin to diverge as soon as the individuals start to develop separately — something already happening in the womb. A true duplicate of an adult would require a mapping of every atom in the body, which can now more or less be done with Nuclear Magnetic Resonance Imaging (NMR or MRI) technology, and then a duplicate set of such atoms being assembled in precisely the same way, something rather further from present technology. This would not be a “clone,” as presently understood, since a clone is only genetically identical. A clone would not have the same memories as the original individual, and it would be no more and no less like the original than an identical twin would be.

Could I be replaced with such a complete duplicate — every atom, not just genetically identical — it would think that it was me. But clearly it would not be me, especially if I were not destroyed in the replacement and continued to exist off somewhere else. We can imagine that such complete identity might produce a being that would simply see itself as existing in two places at once, but this would require some kind of communication; and that would require the existence of some kind of extrasensory or paranormal connection between the two bodies, which is not now part of established science. Without such paranormal communication, the identical individuals would each think of themselves as the original individual, although only one of them would be right; and they would immediately begin to diverge as individuals because of differing experiences.

So what would be the difference between the two individuals? Well, they would exist in different spatial locations, and they would consist of different, albeit identical, atoms — and it is a postulate of quantum mechanics that all particles of the same kind are absolutely identical. I know what it would mean for me not to be that other individual, since it would not be part of my consciousness. However, what if I were to be instantaneously destroyed and replaced with that individual, so that there was, to all appearances, a spatial continuity between us, and a material continuity since, as noted, identical material particles really are identical (there is, according to quantum mechanics, absolutely nothing about them that would enable us to tell them apart). If that individual would still not be me, then there would have to be something else about me that makes me myself apart from physical content, memories, and spatial continuity. In other words, I can perform the thought experiment that would remove “me” from my body, leaving behind an individual that looked, thought, and felt like me, but was not. It would simply not have my consciousness, but another one, which could then ask over again why it is itself and not someone else.

This same kind of thought experiment can be run the other way around: What would it mean for me to be someone else? I can easily imagine suddenly waking up and having another body. Franz Kafka wrote a famous story (“The Metamorphosis,” 1915) in which someone wakes up and has turned into a cockroach. I can also imagine suddenly losing my memory and not remembering who I am. This actually happens to people occasionally. It is also possible to imagine, as in the science fiction movie Total Recall (1990), that the memories of a different person have been put into me, and I wake up, not just not remembering who I am, but actually thinking that I am someone else. Combining these would produce a very dramatic effect: I might wake up both with a very different body and thinking and believing that I am a very different person. If this left me with at least the same brain, however, we would have no difficulty imagining how this could still be “me” in some accountable sense — it would still be my brain regardless of how the body around it might change or what kind of memories might be scrambled or reprogrammed in it. Interestingly, however, our own brain is usually something that we never experience; so were body and memories to be changed, it would be difficult to verify our personal continuity short of neurosurgery. From an internal point of view, and an external one for most practical purposes, everything would be different.

Now if I imagine body, memories, and brain to be replaced, then it would be easy to say that the result could not possibly then be me. However, it is still possible to imagine that the resulting individual could be me, and this act of imagination has actually occurred in multiple world religions for centuries: it would still be me if it were the same immaterial soul. Thus, if I were to believe in reincarnation, I would actually think that I have been innumerable different persons in the past, all with different bodies, memories, and brains. As Krishna says to Arjuna in the Bhagavad Gita: “I have been born many times, Arjuna, and many times has thou been born. But I remember my past lives, and thou has forgotten thine” [4:5, Juan Mascaró translation, Penguin Classics]. Krishna, implies, of course that memories of past lives are retained by the soul. This is not necessary to the thought experiment. It is possible to imagine a soul that does not carry memories but still carries an identical consciousness that would distinguish Arjuna from another individual physically and mentally identical. Since Arjuna (and most of us) does not seem to remember any past lives, this is what is given in experience anyway.

What the thought experiments demonstrate is a truth of metaphysics that the same attributes can belong to different individuals, or in other terms that an individual as an individual cannot be exhaustively defined by abstract predicates. Thus, bodily features, memories, personality, etc. cannot uniquely determine an individual; so I cannot identify myself as an individual by any such qualities. This metaphysical principle has only been disputed by philosophers like Leibniz, who postulate the identity of indiscernibles, that individuals that cannot be told apart are actually the same individual. But such a postulate only works for Leibniz because he denies the existence of space, which can serve to distinguish otherwise identical individuals.

The spatial separation of otherwise identical individuals also can be interpreted to mean that the individuals consist of different quantities of matter. To philosophers as diverse as Aristotle, Descartes, Spinoza, and Schopenhauer, different space and different matter are ontologically identical conditions: Space itself, in effect, is matter. This may now be restated in terms of quantum mechanics: If identical subatomic particles are postulated as absolutely identical by quantum mechanics, then spatial separation, again, is the only thing that individuates identical particles as materially different.

In the thought experiments on my personal identity, spatial or material difference might seem to do the job. An identical copy of me would be spatially and materially different, and if I were replaced by an identical copy, however quickly, it is still possible to imagine that it is materially different, even if instantaneously placed in the same space. However, such a transference would result in no externally ascertainable difference whatsoever, which sounds somewhat paradoxical if were are to say that the “matter” is different. In these terms “matter” must actually be defined in such a way that it is not materially or empirically distinguishable from other matter. Another postulate of quantum mechanics is that if two things cannot be in principle distinguished, then they are the same thing. The only thing that can distinguish identical particles is their spatial location. Thus, if we say that two individuals consist of identical particles and cannot be spatially distinguished (because they are temporally contiguous in the same space), quantum mechanics would then judge that they are the same individual. That there would be a temporal difference doesn’t help, since there is no empirical criterion by which it could be determined whether the “matter” has been switched from one moment in time to another or not. The only way in which we could then say that an identical individual could replace me instantaneously in the same space and still not be me is to require that there be a form of “matter” that is not accessible to physical science. A form of “matter” not accessible to physical science, however, would not be “matter” in any familiar or common sense meaning. An immaterial substance standing in the place of what we would ordinarily call “matter,” however, would more easily be called the “soul.”

If quantum mechanics loses track of matter by only using space to individuate identical particles, the thought experiment of me becoming a different individual contrariwise loses track of space and is only able to use matter for individuation. Thus, I can imagine instantaneously acquiring a different body, different memories, and also finding myself in a different place. If that is nevertheless still me, with my consciousness, it would have to be because the “matter,” or the substantial substrate of my self, was moved to that new location, even if nothing else moved by way of the contents and characteristics of my physical and mental identity. Since such “matter” would then be inaccessible to physical science, it would be reasonable to call such a substantial substrate “immaterial”; and an immaterial substance would reasonably be the “soul.”

It may help to recall what it would mean to say that I could find myself with a different body, a different mind, and in a different location and still be me. It would mean that the conscious existence that I experience now, the conscious existence that seems to disappear in sleep, and which I imagine, or suspect, or fear may simply become nothing in death, can still be imagined as the same conscious existence even if what appears in it is a different body, a different mind, and different place. Thus, I have not become nothing and can still be me, even if I seem to be someone else, cannot remember my old self, and have appeared in a different place. This conception of conscious existence as perfectly divorced from, and so possibly perfectly empty of, content first occurs in the Upanishads, especially the great Brhadâranyaka and Mândûkya Upanishads. Advaita Vedânta then concludes it is only the Self (Âtman) that has substantial, independent existence, while physical objects only exist as illusory appearances in consciousness.

Although “matter” in the senses examined, whether physical or immaterial, is a metaphysical conception that is not accessible to physical science, the device of thought experiments to examine these issues is a perfectly legitimate procedure, not only for philosophy, but even for physical science itself: Einstein’s entire theory of Relativity was based on his own thought experiments. Thus, the basic question here, “Why am I myself, rather than someone else?” is no more dismissible than Einstein’s question about what a light wave would look like if we were moving at the velocity of light with it. The paradox, however, of ending up with a definition of “matter” that abstracts from it all identifiable qualities was not lost on Buddhism, which rejected the idea of a substantial substrate to anything. Like Hume, Buddhism adopted a kind of empiricism where the very conception of substance, whether material or immaterial, did not qualify. However, that produced its own paradoxes, since Buddhism, like Hume, could not then account for the duration in time of objects or persons. Much of Buddhism accepted the doctrine of “momentariness,” that individual objects do not abide for more than a moment, but this is considerably more paradoxical and counter-intuitive that the duration of a substantial substrate. What the Buddhist paradoxes show us is that the substrate, however intangible, is not an unnecessary hypothesis — without it, as a synthetic ground a priori (as Kant would put it), the duration of individuals cannot be accounted for.

Instead, I must appeal to the doctrine of The Origin of Value in a Transcendent Function. Both kinds of “matter” are conceptions of “Negative Transcendence,” the emptiness of existence over and above the phenomenal content of consciousness. Negative Transcendence has internal and external poles. External transcendence then corresponds to physical substance, which in terms of quantum mechanics, as we have seen, is functionally identical to space itself. Internal transcendence is then the substrate for the sense of personal identity that has been examined here in the “thought experiments on the soul.” The question left open in The Origin of Value in a Transcendent Function was in what way internal and external transcendence corresponded to each other.

Now it appears that internal and external transcendence must in an important sense be independent of each other, since external transcendence, as space, cannot account for personal identity from an internal point of view, and internal transcendence, as the “matter” of personal identity, varies independently of space and what can be accessed by physical science. Thus, for there to be personal identity, there must be more than just space and external transcendence. Such a conclusion, however, does not produce a Cartesian Dualism of material and immaterial substances existing in the same logical space, for internal and external transcendence are kept ontologically apart. They are only united through “Positive Transcendence.” Negative Transcendence, in other words, cannot be added as a transcendent object to the order of phenomenal objects. Transcendent objects are subject to the Kantian Antinomies. Rather than being added as a transcendent object to phenomenal reality, internal transcendence casts a “shadow” of Positive Transcendence on phenomenal objects: the numinosity of the self or soul in religious conceptions, or even just the “supernatural dread” associated with dead bodies or cemeteries.

The question, “Why am I myself, rather than someone else?” then, cannot be answered just with natural objects. It can only be answered with transcendence. But transcendence appears in the phenomenal world as the numinous quality of natural objects. This may be called the “soul.” The soul, as an independent, transcendent object, however, cannot be said to be established by this argument. The fact that Buddhism rejects such an object is an important clue that it is subject to the undecidability of a Kantian Antinomy. There is no doubt, on the other hand, of the numinosity of persons in Buddhism, especially as they become Bodhisattvas and Buddhas, and of the reality of karma and reincarnation, despite the denial that reincarnation is the transmigration of a substantial self. Buddhist doctrine thus expresses the paradox of Negative Transcendence as an existence which nevertheless cannot be placed as an object in conceivable (i.e. phenomenal) reality. Later this would be conceived as the “Buddha nature” of individuals, an idea that, not surprisingly, set off controversy about whether this involved a “substantialist heresy” or not. It is, indeed, a fine line, although easily drawn with the theories of Negative and Positive Transcendence.

As noted, it is a postulate of quantum mechanics that subatomic particles in the same quantum states are absolutely identical in characteristics. Although many have believed that Einstein vindicated Leibniz’s view of space, as relative, over Newton’s, this feature of quantum mechanics decisively contradicts Leibniz, for whom space does not exist and objects that are indistinguishable from each other are identical to the same thing. But indistinguishable electrons are not identical to the same thing. They are distinguished from each other by their locations in space (although their possible locations may be summed in the wave function). Leibniz, of course, could respond that what makes the electrons different is their history, and their relationship to other objects. However, while Leibniz believed that his “monads” contained their history, and a representation of their relationships, within themselves, this is not the case for electrons. Indeed, quantum mechanics rules out any such things as “hidden variables.” With an electron, what you see is what you get. And since Leibniz’s monads don’t actually interact with each other, the only terms of their history and their relationship with other objects are their motions and relationships in space. If space does not then exist, monads actually have no history and no relationships.

If we allow that identical objects, however, are distinguished by their locations in space, location in space will not work to account for identity. That is because, as an object moves, it comes to be at a different location. So if different locations serve to distinguish different objects, why does not a object become a different object by moving and coming to be in a different location? This poses a grave difficulty for the theories of matter in Descartes and Spinoza, where matter is all but indistinguishable from space itself. But space does not move around, while matter must move around, to maintain its identity. On the other hand, it is not clear that fundamental particles in quantum mechanics possess any “matter” in the traditional, substantial sense. Energy turns into electrons and positrons. Electrons and positrons collide and turn back into energy. What we see is a collections of attributes, or quantum numbers — mass, charge, spin, etc. — that looks like nothing so much as the “aggregates” (skandhas) of non-substantial existence in Buddhism.

Unfortunately, the denial of substance in Buddhism is intended to effect a denial of identity. If the problem is accounting for the identity of an object or sub-atomic particle from moment to moment, Buddhist metaphysics is specifically designed not to do this. The result is an effective thought experiment in what is required for identity: The “aggregates” are not enough. If an electron has an enduring existence, something has the mass, charge, spin, etc. that characterize it. And if the electron, as electron, ceases to exist, neither Parmenides nor Democritus would be surprised to learn that it does not simply become nothing. Mass/energy is conserved, and there are particles that carry them away. Matter will not be a Cartesian fixed quantity of “stuff” that no transformation can alter, but it will represent a durable continuity of identity, which carries the quantum attributes and can merge with other objects or itself divide into new objects.

This, indeed, is more an Aristotelian than a Cartesian view of matter. Perhaps the only difference is that Aristotelian matter only accounts for different individuals of the same kind. Something unique of its kind (sui generis) doesn’t need matter and can exist as pure form (like God or the celestial intelligences). In the argument here, however, material substance does not merely account for different things of the same kind, or for different individuals that are otherwise identical in every way, but also for the identity of anything with itself from moment to moment — and Aristotle’s substance (ousia), after all, was in the form, not the matter. A Buddhist analysis of Aristotle’s God would be that it has no self, no identity, and duration. Probably not what Aristotle wanted to say, but then he is vulnerable to the critique, since for “substance” he can only offer attributes, i.e. the form.

While Aristotle thought of form as substance, it might be noted that a curious thing happened to the terminology in the translation from Greek to Latin. Ousia is from the participle of the verb “to be” in Greek. Thus, it looks rather like essentia, “essence,” in Latin, which is from the infinitive of the verb “to be” (esse). Substantia, “substance,” itself, is entirely different, meaning to “stand under.” There is a word that means “stand under” in Greek, and that is hypokeimenon. Aristotle does not use that synonymously with ousia. He applies it, as it happens, to matter. Perhaps it would be better to translate ousia as “essence,” in which case we could take substance, the underlying thing, as always being what Aristotle associated with matter. His reluctance, however, is understandable, since he thought of matter as mere power or potential, which would disappear in God. By the time we get to St. Thomas, of course, that idea of a powerless God was unappealing.

If our concern then becomes personal identity, will the identity of material substance account for that? As I have argued, no. In physical terms alone, we know that there is a turnover of matter in our bodies. I believe that after 20 years or so, all the matter in our bodies is supposed to be different. A defendant in a legal case once even tried to argue that he was literally not the same person who had committed the crime, some twenty years plus in the past. His argument was not allowed as, indeed, we trace personal identity across that transformation. With the material objects, this can indeed produce some paradoxical results. The Stoics noticed that in their day the ship kept at Athens, which was supposed to have born Theseus to Crete, had finally been repaired so much that every single plank and other part of it was no longer original. Was it the “same” ship? In a way yes, and in a way no. With material objects, the less the original material, the less it is the original thing. There is no such ambiguity with people. And we can ask them.

Similarly, we can use the thought experiments detailed above. We can think of ourselves persisting even through transformations in body, memories, and everything else. We can even, as it happens, think of ourselves persisting through absences of consciousness. Indeed, we do that every day, as we awake from sleep. This would be challenging for Descartes, for whom the soul was essentially thinking, or for Advaita Vedanta, where the self (âtman) is essentially conscious. But it is not really a problem — consciousness is not essential to identity when the identity of consciousness from moment to moment must itself be accounted for. If personal identity requires a substantial substrate different from material existence, our word for it would be “soul.” This would be a different and more fundamental meaning for it than what soul was for the Greeks, the life force, or for Descartes, consciousness (Searle’s “mental substances”).

The remaining problem would be the epistemological one of why we believe there are substances at all. Not only Buddhism, but Berkeley and then, especially, Hume point out that since substances are behind or beneath everything we experience, we are not directly acquainted with them as such. So what is our evidence that there are such things? The Kantian argument of the “possibility of experience” is then that “substance” is a category, like causality, which is an a priori expectation about experience, not something deduced, derived, or proven from it. Our expectation that the existence of objects is durable, separable, and identical is the principle of “substance” by which we organize and understand experience. While Hume himself said that all reasonings about matters of fact are based on the relation of cause and effect, it is obvious enough that many such reasonings are also based on the persistence, independence, and identity of substance.

What Kant would ask, of course, is what we can know about a substantial soul outside the limits of a possible experience. The soul, after all, is not a natural or phenomenal object, and it is difficult or impossible to imagine how it can exist, as a substance, in the phenomenal world. Kant’s answer then is that the limits of possible experience represent the limits of our knowledge of objects, so that we do not know how it is that substantial souls can exist. An immortal soul, which would be immune to the slings and arrows of man and nature, is in that regard an unconditioned reality, the sort of thing that does not appear in phenomenal existence, either for Kant or Buddhism. Yet even Buddhism does not deny that there are unconditioned realities — most importantly Nirvana. Thus Buddhism, which is ultimately neither materialistic nor naturalistic, actually has more in common with Kant than it does with Hume or, for that matter, John Searle. The soul as a numinous reality, is fully present in the numinosity of the Buddhist Arhat, Bodhisattva, and Buddha. This is no comfort for the materialist, the skeptic, or the nihilist.

No. 5: "In actuality, this story is probably just a legend; instead, it was this elegant thought experiment that helped prove a very important theory about gravity: no matter their mass, all objects fall at the same rate of speed." True, but only in a VACUUM!

that’s what i heard too! were they on the same page about the setting (vacuum or not)? i’m a bit too lazy to research about it right now…. aristotle is probably not talking about a vacuum if he’s considering drag.

what’s wrong with aristotle’s logic? wouldn’t galileo’s reasoning be a bit off if he tied a small parachute-like contraption to a small rock and threw it off a small building?

i didn’t really pay too much attention in my physics classes so a little enlightenment would be appreciated. =)

Mass has no effect on drag. Air resistance is caused by two things, the number of particles hitting the object, and their velocity relative to the object. This has nothing to do with the objects mass, and so it doesn’t matter whether Aristotle was imagining drag or not, it still has nothing to do with mass.

To give an example, if you drop two bricks, but one is denser than than the other, they will still fall at the same rate. They have the same drag, because they are the same shape and size, but their mass is different.

This is crazy. No one is crazy enough to hook up a brain to a computer. That is cruel and unusual. It is also impossible.

I will give you proof why I am right. If this supposed 'mad scientist' created an artificial world for you, then they would never let you find out about the 'brain in a vat'. They wouldn't want you to figure out what's going on.

Also, think about what you love in life. I'm not talking about material things. I'm talking about the stars in the sky, or when flowers are blooming in spring. You can't imagine them being artificial.

Think about yourself. You KNOW you are real. I am real. We are all real.

Most importantly, God created the world for us, not a mad scientist. Read the book of Genesis in the Bible to find out how the world was created.

Please don't let yourself be brainwashed by this 'brain in a vat' nonsense. You are too smart for that.

Don't let yourself be brainwashed by the Bible. Most of the stuff in the Bible is ten times more far-fetched than this stuff. And if you are going to take the book of Genesis seriously, then you have to take it ALL seriously, which is just ridiculous considering the Bible contradicts itself every other page.

Well, all sensory experience takes place in the brain. I'm afraid that you just don't know that you aren't a brain in a vat and that your Bible is just a byproduct of the evil scientists attempts to coerce into believing your reality is real.

I KNOW I am real. I DO NOT know that you are real, nor do you KNOW that I am real. However, because the claim that I exist is not so extraordinary, it takes very little faith to take this statement as truth.

You need to re-read the Cartesian method of doubt if you don't believe it makes logical sense, because I ASSURE you it does. Descartes was an extremely intelligent man and if you haven't read any of his work, it's pretty disrespectful to discard it as "nonsense."

I actually took my time to read the bible before I discarded it as nonsense.

Your "proof" also doesn't make any sense. If you are a brain in a vat…why would the scientist care whether you knew of the possibility of you being a brain in a vat? To you, this simulated reality is 100% convincing and as "real" as the "real" universe.

"I will give you proof why I am right. If this supposed ‘mad scientist’ created an artificial world for you, then they would never let you find out about the ‘brain in a vat’. They wouldn’t want you to figure out what’s going on."

I'm still waiting for your proof. How about you read Carl Sagan to find out how the world was created. I mean, you put your faith in a document written by man as it is. Why not put your faith in a document written by a smart one? As 'aquiredthoughts' says, the bible contradicts itself every other page. It's obviously a load of rubbish.

"Also, think about what you love in life. I’m not talking about material things. I’m talking about the stars in the sky, or when flowers are blooming in spring. You can’t imagine them being artificial."

First off, our brains process all those things, including our emotional reactions to them.

"Most importantly, God created the world for us, not a mad scientist. Read the book of Genesis in the Bible to find out how the world was created."

Second, what you're arguing is that no "mad scientist is controlling us," but that God created everything that we see, hear, touch, experience, etc… Therefore, I could argue that God is the mad scientist. Essentially, God and this "mad scientist" are creating human beings in a certain way, giving them free will and the capacity to gain knowledge. The only difference is believing in God is much more socially accepted, and a person gets a reward at the end of their lifespan if they are a good person. The scientist, however, is given a negative and "cruel" connotation.

If you believe that this theory is "impossible," then what makes God so much more likely? Granted, I am not saying I believe in either option necessarily; however, I do believe that blind faith is not nearly as reliable as science or facts.

"Also, think about what you love in life. Iâ??m not talking about material things. Iâ??m talking about the stars in the sky, or when flowers are blooming in spring. You canâ??t imagine them being artificial."

This is all theoretical. Nobody is going to fly side by side a beam of light either. It’s simply brain food.

That’s one of the most illogical comments I’ve ever had the displeasure of reading.

Firstly, you make the assumption that ‘no one is crazy enough to hook up a brain to a computer’. Says who, exactly?

Secondly, you state that ‘they would never let you find out about the ‘brain in a vat”. Once again, how on earth do you know? That’s just a wild guess!

Thirdly, your thoughts are real, for they are being thought. But the stars? Saying you ‘know they are real’ has no rational reasoning behind it. Have you ever been sat down, and thought someone was standing behind you when they weren’t? Or carried on talking to a friend who had stopped a few yards back to do up their shoelaces? In these scenarios, you ‘know’ there is someone next to you – your senses deceive you. Why couldn’t the same occur in every single aspect of life?

As for ‘knowing’ that God exists, I am yet to hear a reason for doing so that does not, somewhere along the line, include a logical fallacy. Please, give me one?

Oh and finally, ‘don’t let yourself be brainwashed….you are too smart for that’. Are you actually a complete moron? It’s a thought experiment. It is actually suggesting that this is the case, only showing you that you don’t actually know if it is or not! Are you, by chance,desperately worried for the infinite number of innocent monkeys, forced into an eternity of typewriting?

These are crazy! Haven't heard but one of them!

"Descartes accounted for this problem with his classic maxim “cogito ergo sum” (“I think therefore I am”)."

You misunderstand Descartes' meaning. He posed that because you couldn't be sure that you are just a brain in a vat, this implicitly states that your mind must exist, because only that which exists can be deceived. "I think therefore I am" is therefore the ONLY statement you can make with 100% certainty. He was not "accounting for [a] problem"

Should read: "He posed that because you couldn’t be sure that you aren't just a brain in a vat"

In fact, he accounts for the demon problem by appealing to God’s existence and that He could never be a deceiver or allow us to be deceived in such a way. The person who wrote this shouldn’t just guess what Descartes said based on what he’s famous for saying, that’s sloppy stuff.

Also, wasn’t Gettier’s thought experiment called the “Sheep-Shaped Rock”? This could just be another philosopher’s version of the experiment however…

#10 – Sure, you're responsible for the death of 1 person, but you are also responsible for saving the lives of five others. And I guess it depends on who the people are. If that person on the other track was your BFF, then you would not pull the lever.

Or, if the lever acts as a dimmer and not just an on-off switch, you can switch it to the halfway position which would cause the train to derail and kill nobody.

#9 – If the farmer openly stated that he knew the cow was in the field, then yes, he technically knew where his cow was – in the field. He just didn't know where in the field.

#8 – Call the bomb squad, evacuate everyone, and throw the terrorist right next to the bomb.

#6 – Evolution is the answer to all your problems.

#5 – I'm pretty sure this one has to do with covalent bonds between atoms. If there exist no covalent bonds between the rope atoms and feather atoms, or the rope atoms and hammer atoms; then the entire hammer+feather+rope cannot be regarded as one system, it must be regarded as three.

#2 – Keyword: quantum mechanics , not real-life stuff. In real life, you can only say "you just don't know", because a partially-dead cat would be like a zombie.

#1 – Kind of like "we don't know if the universe is going backwards in time [shrinking,] not expanding" because if the universe and everything within it was indeed going backwards, our minds would be thinking backwards. Two negatives cancel out, so we'd perceive time as going forwards.

You changed the variables on some of them and then your other answers didn’t really make a whole lot of sense.

#10 the derailing train would kill the driver or whoever was in the train (not like whoever was in charge was even driving the train anyway though…)

You are totally missing the point of most of those you have commented on.

For #10, you are complicating an essentially simple moral question, and I really hope the bit about the lever acting as a dimmer was a joke.

For #8, I hope that too is meant to be a joke.

#6, what are you on about?

#5, utterly missing the point again, it’s not a matter of definition. Or, if you are actually suggesting that by fusing covalently they would suddenly fall faster, you don’t actually understand the theory of gravity…..

If some of those were actually a joke, sorry for missing it, but if not, then dear God read something on just one of these, because that was depressing to read.

If the person on the other track is a House of Representatives Rightwing Teanut, pull the lever, Quick! Pull it even if there’s no one on the other track ;’)

Failed Logic, Mooney.

Number 8 (The Ticking Time Bomb) – look up the movie Unthinkable – http://www.imdb.com/title/tt0914863/

Number 8 (The Ticking Time Bomb) poses the moral question – Is it right to torture one person in order to get information to save many more? BLOODY RIGHT IT IS! I have heard the PC brigade bleat on about Guantanamo Bay and human rights abuses going on there. So what? If it provides any information that stops another atrocity then it is right. "But some of these people may be innocent" some of you might say. My parents told me if i played with matches I might get burnt! Every one of these "innocent people" I have read about have been questionable to say the least. One of these poor, innocent chaps, released to Britain, had used a false name and passport to leave the country before being captured. Whilst I am on my soapbox, and as a British person, why are these detainees always referred to as British citizens (where i live anyway), when in fact they have been born elsewhere (usually in the middle east somewhere).

Torture the one to save the many! TOO RIGHT! If the person being tortured was a direct threat to the wooly hatted, vegetarian, petrol hating, hippy who is ranting about human rights then the hippy wouldn't be a shouting so loudly.

I wonder if you would still hold this view if the person being tortured was your mother, father siblings, children or spouse/girlfriend or even YOU?

Kleanthis01 you make the argument that our outlook is changed if our loved ones are the ones in question,but this argument works both ways. What if it was your family/friends/significant other who was trapped in said city?

Haha you sound like the byproduct of a brainwashed fascist authority paradigm… Meaning, you’re completely controlled by fear and willing to dispense of inalienable human rights for safety. This is the greatest illusionary sham ever created. Every communist and totalitarian system stems from the disposition that it knows what’s in your best interest. So long as you let fear control you, your life’s efforts will remain futile.

Except the CIA and State Dept Created both Iran and Al Quada in their current form. Maybe their leaders should be sent to Guantanamo.

I like the Brain In A Vat. Sure, The Matrix ripped off this idea, but it's still a good one. How do we know we're not just some evil entity's toy, a brain in a vat?

The only thing you can be sure of is that we exist. Yes, life may be just an impression, a lucid dream. But in order for you to be deceited, you have to exist. So life will always be true, even if it is a lie.

But you could have only just started existing your memories lies and time is infinetly divisible so your life could be infinetly small

but an infinitely small life, since time is infinitely divisible, is also infinitely large. As long as life exists as a duration, not just a single moment in time, than between its beginning and end there are an infinite number of moments

“since time is infinitely divisible,” Except, of course, that time is NOT infinitely divisible. It occurs in discrete units, just like all other fundamental phenomena of the universe. and the smallest possible measure is one Planck Time (Google it). One Planck Time is 0.000000000000000000000000000000000000000000054 seconds. Anything shorter than that is meaningless, in physics. So there are a finite number of moments between the “beginning” and the “end” of any event, including life. Your days are numbered, and so are your Planck Times. 🙂

You can’t hold that life is illusory then try to use scientific knowledge gained through that illusion. For all you know, Planck’s time unit is a complete fabrication. The only types of thought that hold weight in an illusory world are thoughts which do not depend upon that world for evidence (such as formal logic or abstract thought). Since Planck’s time unit is based upon the travel of light in a vacuum it is therefore dependent upon evidence in this possibly illusory world. Thus, it is inadmissible as evidence to disprove that time is infinitely divisible.

The “brain in a vat” is easily debunked, some of the more interesting brain teasers should have been ranked higher, as they have far more moral, scientific, or philosophical conundrums than the “brain in a vat” theory. Let’s say my brain is a simulation, furthermore, let us say that it has been since birth… take 10 children, ask them to identify a shape in a particular cloud, chances are you will receive 10 different variations in a single cloud. Now let’s go back to just me and assume I am the only one affected by the “vat,” that a particular simulation would already determine that I would think of this idea… the “program” could never account for the answers I would get from 10 random children seeing 10 very different shapes. Yes we age and it can become increasingly easy for us to believe that the world may already be run by “Skynet,” but as long as we have the abilithy to find art and random beauty in nature, I find it very hard to believe that a simulation could ever “predict” how we would perceive a natural phenomenon as witnessed not only by our own “computer generated psyche,” but how the same phenomenon would be witnessed completely different by the same “vat” mentality. Witness the world you live in through the eyes of a child and rest assured, there is no mechanical program that could ever judge how we perceive our world through the eyes of a child.

I think you are not taking into account that the same way the evil scientist is making trees appear for you to interact with he could very easily make ten kids and whatever the results of that experiment to be. Thus not proving anything but that you are now further convinced the matrix is real in your vat.

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7 thought experiments that will make you question everything

• Thought experiments provide a platform to examine abstract concepts in a playful and imaginative manner. 
• The best thought experiments challenge our beliefs and offer fresh perspectives on how the world operates by presenting hypothetical situations. 
• These hypothetical situations are often constructed in extremes so we can see how certain ideas might play out — all without suffering any real-world repercussions.

Thought experiments are among the most important tools in the intellectual toolbox. Widely used in many disciplines, thought experiments allow for complex situations to be explored, questions to be raised, and complex ideas to be placed in an understandable context. Here are seven thought experiments in philosophy you might not have heard of, complete with explanations of what they mean and what questions they raise.

Written by Donald Davidson in 1987, this thought experiment raises questions about identity.

Suppose a man is out for a walk one day when a bolt of lightning disintegrates him. Simultaneously, a bolt of lightning strikes a marsh and causes a bunch of molecules to spontaneously rearrange into the same pattern that constituted that man a few moments ago. This “Swampman” has an exact copy of the brain, memories, patterns of behavior as he did. It goes about its day, works, interacts with the man’s friends and is otherwise indistinguishable from him.

Question: Is the Swampman the same person as the disintegrated fellow?

Davidson said no. He argues that while they are physically identical and nobody would ever notice the difference, they don’t share a casual history and can’t be the same. For example, while the Swampman would remember the friends of the disintegrated man, it never saw them before. Another person saw them and the Swampman just has his memories.

There are objections to the idea that the two characters in the story are different. Some argue that the identical minds of Swampman and the original person mean that they are the same person. Others, like philosopher Daniel Dennett, argue that the entire experiment is too far removed from reality to be meaningful.

This raises problems for teleportation as seen on Star Trek and for those who want to download their brains into a computer. Both cases rely on one version of you being created and one disappearing, but is the second version of you still you ?

Thompson’s violinist

This one was written by Judith Thomson in her 1971 essay A Defense of Abortion . She writes: “You wake up in the morning and find yourself back to back in bed with an unconscious violinist. A famous unconscious violinist. He has been found to have a fatal kidney ailment, and the Society of Music Lovers has canvassed all the available medical records and found that you alone have the right blood type to help. They have therefore kidnapped you, and last night the violinist’s circulatory system was plugged into yours, so that your kidneys can be used to extract poisons from his blood as well as your own. If he is unplugged from you now, he will die; but in nine months he will have recovered from his ailment, and can safely be unplugged from you”

Question: Are you obligated to keep the musician alive, or do you cut him loose and let him die because you want to?

Thompson, who has several excellent thought experiments to her name, says no. Not because the violinist isn’t a person with rights, but rather because he has no right to your body and the life-preserving functions that it provides. Thompson then expands her reasoning to argue that a fetus also lacks the rights to another person’s body and can be evicted at any time.

Her argument is subtle, however. She doesn’t say you have a right to kill him, only to stop him from using your body to stay alive. His resultant death is viewed as a separate, yet related, event that you have no obligation to prevent.

The Veil of Ignorance

This experiment was devised by John Rawls in 1971 to explore notions of justice in his book A Theory of Justice.

Suppose that you and a group of people had to decide on the principles that would establish a new society. However, none of you know anything about who you will be in that society. Elements such as your race, income level, sex, gender, religion, and personal preferences are all unknown to you. After you decide on those principles, you will then be turned out into the society you established.

Question: How would that society turn out? What does that mean for our society now?

Rawls argues that in this situation we can’t know what our self-interest is so we cannot pursue it. Without that guidepost, he suggests that we would all try to create a fair society with equal rights and economic security for the poor both out of moral considerations and as a means to secure the best possible worst-case scenario for us when we step outside that veil. Others disagree, arguing that we would seek only to maximize our freedom or assure perfect equality

This raises questions for the current state of our society, as it suggests we allow self-interest to get in the way of progressing towards a just society. Rawls’ ideas about the just society are fascinating and can be delved into here .

The Experience Machine

Robert Nozick came up with this thought experiment, which appears in his book Anarchy, State, and Utopia.

Imagine that super neuroscientists have created a machine that can simulate pleasurable experiences for the rest of your life. The simulation is ultra-realistic and indistinguishable from reality. There are no adverse side effects, and specific pleasurable experiences can even be programmed into the simulation. Regarding pleasure experienced, the machine offers more than is possible in several lifetimes.

Question: Do we have any reason to not go in?

Nozick argues that if we have any reason to not get in then hedonistic utilitarianism, the idea that pleasure is the only good and that we ought to maximize it, is false. Many people value having real experiences or being a person who does things rather than dreams about doing them. No matter what the reason, if you don’t go in you can’t claim pleasure is the only good, and Nozick thinks most people won’t go in.

There are counter-arguments, however. Some hedonists argue that people really would go into the machine or that we have a status quo bias that leads us to treat the reality we are currently in as more important than other, better ones. In either case, the experiment does present us with a problem for those who argue we only want pleasure.

Mary’s Room

In 1982, philosopher Frank Jackson proposed this thought experiment that raises questions about the nature of knowledge.

Mary lives in a black and white room, reads black and white books, and uses screens that only display images in black and white to learn everything that has ever been discovered about color vision in physics and biology. One day, her computer screen breaks and displays the color red. For the first time, she sees color.

Question: Does she learn anything new?

If she does, then it shows that qualia, individual occurrences of subjective elements of experience, exist; as she had access to all possible information other than experience before she saw the color but still learned something new.

This has implications for what knowledge and mental states are. Because if she learns something new then mental states, like seeing color, can’t be described entirely by physical facts. There would have to be more to it, something subjective and dependent on experience.

If she doesn’t learn anything new, then we would have to apply the idea that knowing physical facts is identical to experiencing something everywhere. For example, we would have to say that knowing all about echolocation is similar to knowing what it is like to use it.

This experiment is unique of the ones on this list as the author later changed their mind and argued that Mary seeing red doesn’t count as evidence that qualia exist. However, the problems posed by the experiment remain widely debated.

Buridan’s Ass

Variations on this experiment date back to antiquity, this formulation was named after the philosopher Jean Buridan, whose views on determinism it ridicules.

Imagine a donkey placed precisely between two identical bales of hay. The donkey has no free will, and always acts in the most rational manner. However, as both bales are equidistant from the donkey and offer the same nourishment, neither choice is better than the other.

Question: How can it choose? Does it choose at all, or does it stand still until it starves?

If choices are made based on which action is the more rational one or on other environmental factors, the ass will starve to death trying to decide on which to eat- as both options are equally rational and indistinguishable from one another. If the ass does make a choice, then the facts of the matter couldn’t be all that determined the outcome, so some element of random chance or free will may have been involved.

It poses a problem for deterministic theories as it does seem absurd to suppose that the ass would stand still forever. Determinists remain split on the problem that the ass poses. Spinoza famously dismissed it while others accept that the donkey would starve to death. Others argue that there is always some element of a choice that differentiates it from another one.

The life you can save

This experiment was written by famed utilitarian thinker Peter Singer in 2009.

Imagine that you are walking down the street and notice a child drowning in a lake. You can swim and are close enough to save her if you act immediately. However, doing so ruins your expensive shoes. Do you still have an obligation to save the child?

Singer says yes, you have a responsibility to save the life of a dying child and price is no object. If you agree with him, it leads to his question.

Question: If you are obligated to save the life of a child in need, is there a fundamental difference between saving a child in front of you and one on the other side of the world?

In The Life You Can Save, Singer argues that there is no moral difference between a child drowning in front of you and one starving in some far off land. The cost of the ruined shoes in the experiment is analogous to the cost of a donation, and if the value of the shoes is irrelevant than the price of charity is too. If you would save the nearby child, he reasons, you have to save the distant one too . He put his money where his mouth is, and started a program to help people donate to charities that do the most good .

There are counter-arguments of course. Most of them rely on the idea that a drowning child is in a different sort of situation than a child who is starving and that they require different solutions which impose different obligations.

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• Thought experiment

A thought experiment (Ger. Gedankenexperiment ) is (typically) an experiment that can't, or shouldn't, actually be performed, but that still leads to interesting results or understanding just by its consideration. Sometimes the German term Gedankenexperiment is used in English.

Famous scientific thought experiments [ edit ]

• Schrödinger's cat is perhaps one of the more famous thought experiments where a cat is said to be both dead and alive in a demonstration of quantum superposition. Often misunderstood. It was conceived to question the Copenhagen ("superposition") interpretation of quantum mechanics, by showing dramatic macroscopic consequence of a quantum effect under that interpretation. It was not meant to assert that macroscopic superposition is a necessary consequence of quantum theory.
• Quantum suicide (and also quantum immortality ) is a variant on the cat-in-a-box experiment that tests the implications of the " many worlds " and Copenhagen interpretation of quantum mechanics, which are only interpretations and currently have no practical differences.
• Maxwell's demon is a famous thought experiment questioning the second law of thermodynamics .
• Newcomb's Problem is a thought experiment popular with those working on decision theory. In the experiment you have two boxes and an intelligent being who can predict your move perfectly and alters the content of the boxes based on that prediction; leading to a paradox of what decision strategy gives you the most profitable result.
• The Twins Paradox of special relativity . Concerning the predicted differential ageing of twins reunited after one has been on a rocket trip. Often misunderstood. The paradox is NOT simply that the reunited twins are no longer the same age (that is just a non-intuitive consequence of Relativity theory). The paradox is meant to be that since velocity is relative, either twin may regard himself as the stationary one, so when they are reunited each twin must paradoxically find the other twin older than himself. There is no paradox because the twins are NOT both free to regard themselves as permanently stationary — the rocket twin "knows" he has undergone acceleration. The simple rules of Special Relativity do not apply in accelerated frames of reference.
• The Grandfather Paradox relating to time travel postulates what happens if you go back in time and kill your grandfather, thus preventing you from being born and preventing you from going back in time and killing him, which would leave him alive so that you could go back and kill him, preventing you from existing in order to go back in time and kill him, so he'd survive and you'd live, allowing you to go back in time and kill your grandfather… just smile and nod.
• Newton's Cannonball , used to demonstrate how orbits work. Best demonstrated with a Flash game.

Less famous scientific thought experiments [ edit ]

• The Buttered cat theory treats the problem of whether a falling cat with a slice of buttered toast attached to its back can ever hit the ground. Cats always land on their feet and toast butter-side down, creating a paradox.

Famous philosophical thought experiments [ edit ]

• AI-box experiment , an experiment devised by Eliezer Yudkowsky to show that a suitably advanced artificial intelligence can convince, or perhaps even trick or coerce, people into "releasing" it — that is, allowing it access to infrastructure, manufacturing capabilities, the Internet, and so on.
• The Chinese room , an experiment challenging the validity of the Turing test . Here, the experiment is intended to demonstrate that a machine that passes the Turing test isn't necessarily conscious.
• The China brain experiment, where the population of China (as it has a large population and can be visualised, in more recent years this could perhaps be called the "India Brain") simulates the neurons within a working brain to demonstrate the emergent nature of consciousness.
• The Trolley Problem , a thought experiment first put forward by Philippa Foot, which distinguishes between "killing" and "letting die". The thought goes that you're standing by a set of tracks as a runaway trolley flies by. If it continues on its course, the five people tied to the tracks will be killed. If it is diverted onto a second set of tracks, the one person tied to the tracks will die. You are in front of the diverting lever and must choose — do nothing, and allow five people to die, or pull the lever, and intentionally kill one person (many variations have been formulated, such as The fat man or The loop variant ).
• Mary in the Gray Room , on which a girl called Mary, who lived her entire life in a black-and-white room learning everything possible about optics and the science of colors, from how light is reflected and absorbed in the cornea to what colors are supposed to represent, leaves her room and actually experiences color for the first time. The discussion is what would be Mary's reaction: would she be surprised or not?
• Russell's Teapot , an analogy proposed by Bertrand Russell , where the existence of a teapot orbiting the sun somewhere between Earth and Mars is proposed, and those who disagree with this idea are invited to "disprove" it (or at least try).
• Brain in a vat : if we really can't know anything, our entire reality can be fake.
• Ship of Theseus : Imagine that a ship which fought a great battle, while captained by the hero Theseus, is stored in a harbor as a museum piece. As the years go by, some of its rotten parts are replaced by new ones until, eventually, all of the parts from the original ship are replaced. If someone were to recover the original parts and rebuild the ship at another location, which of the ships should be considered the true ship of Theseus? Is there a point at which the structure at the first location stopped being the ship of Theseus?
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The past and present of thought experiments’ research at Glancy: bibliometric review and analysis

• Open access
• Published: 07 September 2024
• Volume 3 , article number  142 , ( 2024 )

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• Hartono Bancong 1  

In the development of physical theories, thought experiments play a crucial role. Research on this topic began in 1976 and has continued to the present. This study aims to provide a more complete picture of the progress of thought experiments over the past two decades. To achieve this, this study employs bibliometric mapping methods. A total of 679 published papers were analyzed, including articles (504), conference papers (92), and book chapters (83). This data was retrieved from the Scopus database. The study's findings reveal that research and publications on thought experiments are highly valued and have received significant attention over the past eight years. According to the findings, 90% of the top 20 source titles contributing to thought experiments are from journals in the first and second quartiles (Q1 and Q2). This quartile ranking shows the quality and significant influence of a journal. The geographical distribution indicates that the United States contributes the most to thought experiments research, with 213 documents, 2592 citations, and 47 links. We also identified several prospective keywords that could be the focus of future research, including artificial intelligence, physics education, fiction, God, theology, productive imagination, technology, speculative design, and critical design. Therefore, this study provides a thorough picture of thought experiment research trends and future directions of potential topics that can be the focus of future researchers.

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1 Introduction

Thought experiments (TEs) have a long history in science. Since Ernst Mach, the term TEs, a direct translation of the phrase Gedankenexperimente , has been widely discussed in the philosophy of science [ 18 ]. Thought and experiments are two components of TEs [ 4 , 18 , 29 ]. The thought element involves visualizing an imaginary world based on theory and experience, whereas the experimental aspect entails practical tasks in a physical laboratory, such as manipulating items and related variables. While some authors consider TEs to be mere arguments [ 24 ], others believe TEs are a form of fiction since their function is comparable to literary fiction in that both have a narrative framework by creating scenarios of occurrences from beginning to end [ 13 , 22 ]. However, unlike fiction, which frequently provides contradictory discourses, we believe that TEs should be logically and conceptually cohesive. TEs are structured imaginative actions based on the theory and experience of thought experimenters to achieve certain goals.

The contributions of TEs to the growth of scientific theories, particularly in physics, are essential. Physicists have employed TEs several times throughout history to either come up with new hypotheses or disprove previous ones. As the most representative examples, Newton used the TEs of cannonballs to support his hypothesis that the force of gravity is universal and the principal force of planetary motion, or Galileo used the TEs of free-falling bodies to disprove Aristotle's theory of gravity, which stated that the speed of falling objects is proportional to their weight. Galileo’s falling body, Newton's bucket and cannon, Maxwell's demon, and Schrodinger’s cat are just a few of the well-known TEs in physics [ 4 ]. These are only a few examples of the significant role TEs played in the development of scientific theories.

In the past 10 years, several works have studied TEs from the perspectives of history and philosophy of science [ 7 , 8 , 10 , 30 , 33 ]. Because most existing historical work on TEs focuses on individual TEs or individual accounts of TEs, reassessing the history of the philosophical debate on TEs becomes essential [ 33 ]. In the philosophy of science, historical debates regarding interactions between various philosophers or philosophical explanations across time in developed TEs are sometimes disregarded. Several studies have also used TEs as an imaginative tool in the classroom to teach science subjects. Velentzas and Halkia [ 37 ], for example, used TEs from Newton's Cannon to teach satellite physics. They then assert that TEs, as a teaching tool, can assist students in strengthening their syllogistic abilities and help them conceive scenarios beyond their everyday experience [ 37 ]. El Skaf and Palacios [ 12 ] have also systematically analyzed the epistemic role of TEs from Wheeler's demon and Geroch's engine, which gave rise to black hole thermodynamics. Recently, Bancong et al. [ 2 ] reported that physics teachers in Indonesia have a high awareness of the importance of TEs in learning physics, especially atomic theory and relativity, even though they lack skills in the pedagogic aspects of TEs. Therefore, Indonesian physics teachers also suggest using technology such as virtual reality to help visualize an imaginary world when performing TEs.

Although a number of studies on TEs from various perspectives have been conducted, no study has yet completely examined this field to look at the trend of this topic in recent years. Therefore, it becomes essential to conduct a bibliometric study of TEs over time based on authoritative databases like Scopus. Because of Scopus's comprehensive coverage of scholarly articles in the field of education [ 23 , 27 , 34 ], it was chosen as the database for this study. Scopus is also a popular resource for bibliometric research [ 23 , 28 ]. For this reason, we use data sources from the Scopus database to carry out the bibliometric method. Our study covers journal articles, conference papers, and book chapters from the last 20 years to provide a more complete view.

To highlight the significance of TEs research, we compare its growth to other scientific topics. While many scientific fields have seen growth over the past two decades, TEs research has also shown a unique and sustained increase in interest and publications. This trend contrasts sharply with the decline in research focus on traditional physics experiments [ 41 ]. Similarly, other topics in physics education, such as methodological issues, textbook analysis, and pre-service physics teachers, are also experiencing reduced research interest [ 25 ]. Additionally, the integration of TEs with emerging technologies, such as artificial intelligence, underscores their evolving relevance and potential for future research [ 21 ].

Therefore, this study aims to provide an up-to-date overview of trends in TEs research. The research questions in this study are as follows:

How is the growth of research output on the topic of TEs over the last 20 years?

Which source titles have contributed the most to the publication of papers on TEs in the last 20 years?

Who are the most prominent authors on the topic of TEs in the last 20 years?

Which countries have published the most articles on TEs over the past 20 years?

What are the most relevant keywords that can be found in the studies of TEs over the last 20 years?

2.1 Research design

This study aims to analyze the trends in TEs research over the past 20 years by using a bibliometric mapping method. To ensure a thorough analysis of recent trends and developments, this study focused on studies published between 2003 and 2022. This period was chosen because of significant advancements in research methodologies and bibliometric analysis tools in the early 2000s, as well as the consistent growth and comprehensive coverage of the Scopus database since that time. Bibliometric analysis is a well-known statistical method for examining and analyzing a large amount of scientific data on a certain topic [ 26 , 39 ]. Metrics studied in bibliometric research include annual publications, source titles, authors, institutions, nations, and keywords, covering data from primary, secondary, and tertiary journals over a specific time period. It should be noted that no ethical approval was required for this study as it did not involve humans or animals.

2.2 Data collection

In this study, data were gathered from the Scopus database ( https://www.scopus.com ). Scopus was chosen because it covers a wider range of documents than any other scientific database [ 23 , 28 , 35 ]. Scopus is the world's largest abstracting and indexing database, with 84 million records containing over 18.0 million open access items, including gold, hybrid gold, green, and bronze, as well as 10.9 million conference papers, 25.8 thousand active peer-reviewed journals, and over 7000 publishers [ 14 ]. In addition, Scopus covers a wider range of educational disciplines than other databases, such as the Web of Science (WoS) [ 23 , 27 , 34 ]. As a result, using the Scopus database enables researchers to shed light on areas that may not be covered in WoS.

Electronic data search and retrieval were conducted on February 25, 2023. Keyword search was set to include title, abstract, and keywords. The keyword search was set to include the title, abstract, and keywords. The combination of search strings, operators, and filters used in this study was TITLE-ABS-KEY ("Thought-experiments" AND "Science" OR "Physics"). Quotation marks were used to focus on documents containing this exact phrase, thus ensuring high relevance to the study's scope. The Scopus database retrieved 898 documents related to these keywords with full bibliographical information, including articles (67.04%), paper proceedings (10.13%), book chapters (10.02%), and other types of documents (12.81%). By using the Scopus filter, other types of publications (12.81%), including review articles, were excluded from the list of documents. The exclusion of review articles was intentional to focus on original research contributions that advance the field of TEs directly. Including reviews could confound the analysis as they often summarize existing research rather than introduce new findings. Therefore, concentrating on the three most prevalent types of documents—articles, conference papers, and book chapters—allowed for a clearer interpretation of trends and patterns in original research outputs over the specified period. Additionally, we limited the year of publication to studies published within the last 20 years (2003–2022) to ensure the relevance and currency of our analysis. After using a filtering process to eliminate papers that did not meet the inclusion and exclusion criteria, a total of 679 articles were identified for bibliometric analysis. These articles included 504 articles, 92 book chapters, and 83 conference papers.

2.3 Data analysis

The data analysis process began with acquiring the necessary raw data by downloading it from the Scopus database in either comma-separated value (CSV) or research information system (RIS) format. For data analysis and visualization, we used VOSviewer and Microsoft Excel. VOSviewer, a sophisticated mapping tool, was employed to create collaborative networks for various variables and keywords, while Microsoft Excel was used for descriptive analysis, such as determining the number of articles published each year and identifying the most prolific source titles.

The network graphs in this study were generated using VOSviewer, based on co-authorship, co-occurrence, and citation data from Scopus. The analysis type focused on the co-occurrence of keywords and co-authorship, with a full counting method. Keywords with a minimum of four occurrences were included. The visualization settings in VOSviewer were mainly default, with the attraction parameter set to 2 and the repulsion parameter set to 0. These settings ensured that the most relevant and frequently occurring terms were highlighted, providing a clear overview of research trends and collaborations in the field of TEs over the past 20 years.

In this study, we explored the most productive publishers, the most referenced articles, the most productive authors, the most productive nations, and author keyword occurrences across time. An analysis of co-authorship and co-occurrence was performed at this stage. The analysis of co-authorship provides insights into the interactions between authors. This methodology was also used for metrics related to countries. For country attribution, we included all the countries of all authors involved in each publication, not just the corresponding author. This method ensures that all co-authors' contributions are acknowledged and provides a comprehensive representation of the global distribution of research. Co-occurrence analysis was employed as a means of investigating current keywords and their interrelationships with other phrases associated with TEs. Within this particular framework, the term “node size” refers to the frequency at which a certain keyword appears in comparison to other words. Additionally, interconnected nodes are visually represented by lines known as connections. The link establishes a connection between two nodes, while the width of the link signifies the intensity or potency of the connection between the aforementioned nodes [ 36 , 39 ].

In the context of network map visualization, nodes that exhibit a high degree of association are categorized into clusters. The clustering of items was performed using the Louvain algorithm, a popular method for community detection in large networks due to its efficiency and accuracy in handling large datasets [ 36 , 39 ]. This algorithm was chosen for its ability to uncover modular structures within large networks, which is particularly useful for identifying distinct research themes and collaboration groups in bibliometric data. Subsequently, a distinct color code was assigned to each cluster, wherein nodes within the same cluster exhibit a high degree of homogeneity. Therefore, this bibliometric mapping approach enabled researchers to discern patterns and emerging areas of interest throughout the timeframe spanning from 2003 to 2022. Figure 1 shows the stages in the process of collecting and analyzing data in this study.

The steps in collecting and analyzing the data

3.1 Statistics analysis

In this analysis, we use statistical data to observe differences in the number of articles published each year. The goal is to determine whether the quantity of publications on the topic of TEs has increased or decreased annually. Figure 2 illustrates the number of papers published over the last 20 years (2003–2022). As we can see, there has been an increase in the interest and attention of researchers, scholars, and experts in researching TEs. The growth started in 2004 and continued until 2006. The number of papers published then fluctuated between 2006 and 2015. The increase started again in 2015 and continued until 2021. The number of publications increased significantly in 2021, with 69 articles published. This growth demonstrates that research and publications on TEs are in high demand and have garnered significant attention globally in the last eight years despite a reduction in 2022. Although studies in this area are still ongoing, these findings indicate an annual growth in the writing and publication of TEs on Scopus.

Number of articles published each year

Statistical data are also used to see the number of source titles that have made the greatest contributions to TEs during the last 20 years. A total of 679 papers have been published from various sources with different types of documents in the form of articles (504), conference papers (92), and book chapters (83). According to statistical data in the Scopus database, publication in journals is very significant in publishing research on the topic of TEs, while publication in proceedings and book chapters with the main scope of TEs is not very significant. Therefore, researchers, academics, and experts are advised to submit their articles focused on TEs to journals rather than proceedings and chapter books. Table 1 lists the top 20 sources of scientific research publications covering the topic of TEs from 2003 to 2022.

As seen in Table 1 , 90% of the source titles contributing to the TEs topic are journals, with only one publishing conference proceedings. Philosophical studies ranks first, with 17 documents published in the last 20 years. This is followed by the AIP Conference Proceedings with 15 documents. The American Journal of Physics, Science and Education, and Studies in History and Philosophy of Science Part A have published 11 documents each. Other source titles, such as Synthese (10), Foundations of Science (9), Physics Teacher (9), Journal for General Philosophy of Science (8), and Philosophy of Science (8), also contributed to publishing TEs topics. Minds and Machines and Physics Education each published seven documents. Erkenntnis, European Journal of Physics, Physics Essays, and Religions each published six documents, Acta Analytica published five documents, while Axiomathes, Boston Studies in the Philosophy of and History of Science, and European Journal for Philosophy of Science each published four documents.

3.2 Bibliometric analysis

3.2.1 contributions of authors.

Table 2 shows the 10 most prolific authors based on the total number of published articles from 2003 to 2022. As shown in this list, Stuart is the most significant author with 7 papers (51 citations), followed by Bancong from Universitas Muhammadiyah Makassar, Indonesia, with 5 papers (15 citations). Following Bancong, Fehige from the University of Toronto, Canada, has also published 5 articles. The majority of Fehige’s research focuses on TEs in the context of religion. In contrast to Fehige, Brown, also from the University of Toronto in Canada, has studied TEs through the lens of history and philosophy of science in several of his works (4 documents, 52 citations). Similarly, Buzzoni (3 documents, 15 citations) and El Skaf (3 documents, 29 citations) from Italy, discuss TEs from historical and philosophical perspectives of science. Meanwhile, Halkia and Velentzas from the University of Athens, Greece, have analyzed TEs thoroughly from an educational standpoint, with the number of documents being 4 and 86 citations.

3.2.2 Contributions of country

In the context of the leading countries, authors from 64 different countries/territories published a total of 679 documents. Table 3 lists the top 20 countries in terms of TE contributions based on the number of papers published. As shown, the United States contributes the most to TEs research, with 213 documents, 2592 citations, and 47 links. The number of papers is about three times that of the United Kingdom, which comes second (75 documents, 1016 citations, and 31 links). European countries continue to hold third to sixth place, with Germany publishing 50 documents with 634 citations, followed by Canada (43 documents, 410 citations, and 17 links), Italy (33 documents, 96 citations, 6 links), and the Netherlands (28 documents, 342 citations, and 12 links). This suggests that countries in America and Europe contribute the most to TEs. The Asian country that has contributed the most to TEs is China, with 18 documents, 286 citations, and 11 links, followed by India (14 documents), Japan (12 documents), and South Korea (12 documents), with 97, 111, and 27 citations, respectively. The three countries below these are European countries, with Austria having issued 10 documents related to TEs with a total of 135 citations, followed by Finland (9 documents, 31 citations) and Spain (9 documents, 47 citations).

3.2.3 Keywords

The results of a keyword analysis can be used in further investigation of the topic at hand. This study employs a minimum threshold of two occurrences of keywords in all research articles that were examined using VOSviewer. Figure 3 displays the 253 authors' keywords detected from 1990, which may be categorized into six distinct clusters. Cluster 1 is characterized by a red color, Cluster 2 by a green color, while Cluster 3 is distinguished by a blue color. In addition, Cluster 4 is characterized by a yellow color, Cluster 5 has a purple hue, and Cluster 6 is distinguished by a light blue shade. Each cluster is comprised of interconnected keywords that are visually represented by the same colors. It is important to note that the size and shape of the node are indicative of the frequency of its occurrences [ 36 , 39 ]. In other words, there is a positive correlation between the size of the node and the frequency of occurrences of these terms. Clustering is employed as a means to gain insights or a comprehensive understanding of bibliometric groupings, whereas image mapping serves the purpose of obtaining a holistic depiction of a bibliometric network.

Network visualization of TEs

Figure 3 shows Cluster 1 (red) with 68 items such as thought experiments, intuition, Science, Kant, Aristotle, Galileo, Platonism, personal identity, theology, fiction narrative, moral motivation, and neuroscience. Cluster 2 (green) consists of 57 categories, such as science fiction, philosophy of science, philosophy of physics, philosophical thought, epistemology, knowledge, scientific reasoning, experiments, models, and realism. Cluster 3 (blue) contains 41 items, such as consciousness, Maxwell's demon, Schrodinger's cat, quantum theory, entropy, uncertainty principle, quantum entanglement, quantum information, quantum physics, and Newton's bucket. Furthermore, cluster 4 (yellow) consists of 30 items: physics education, science education, visualization, special theory of relativity, history of physics, problem-solving, exploration, Einstein, relativity, and falsification. Cluster 5 (purple) consists of 29 items: imagination, ontology, physics, truth time, algorithm of discovery, artificial intelligence, ethics, nanotechnology, fiction, philosophy, and technology. Finally, cluster 6 (light blue) contains 16 categories, including popular science, fictionality, narrative, construction, sensation, a priori, story, Mach, memory, productive imagination, and schema.

Keywords in clusters 1 and 2 have a high number of occurrences and a high total link strength. The term thought experiment ranks first with 85 occurrences and a total link strength of 91. This is followed by the term thought experiment with 60 occurrences, a total link strength of 98, and several other keywords. The high number of occurrences and high total link strength indicate that scientific research publications on the topic of TEs in the 2003–2022 range indexed by Scopus have a strong and direct relationship with these keywords. Table 4 displays the ten keywords with the highest occurrence and overall link strength in the last 20 years on the topic of TEs.

VOSviewer, on the other hand, is also used to visualize the progress of keywords over a certain period. Figure 4 illustrates the overlay visualization of the TEs topic in the time range 2003 to 2022.

Overlay visualization of TEs

Figure 4 depicts the annual distribution of the number of articles containing keywords. The various colors represent the publication dates of the related papers where these keywords appear together. The data in Fig. 4 indicate that the most frequently used topics related to TEs from 2010 to 2014 were quantum theory, ethical naturalism, ethical naturalism, quantum mechanics, scientific discovery, and mental models. Then, from 2014 to 2018, keywords such as scientific reasoning, intuition, science education, computer simulation, history of science, and science fiction began to appear in the TEs topic. The hottest topics in TEs research are shown in yellow color, including fiction, artificial intelligence, God, theology, speculative design, critical design, and methods of case. These findings indicate that these keywords have gained popularity in recent years. It can be concluded that scholars have increasingly turned to research on the mentioned topics in recent years.

4 Discussion

The goal of this study is to use the bibliometric mapping method to examine the trend of studies on TEs during the last 20 years (2003–2022). According to the findings of the study, there has been an increase in the interest and attention of researchers, scholars, and professionals in studying TEs. Although research in this area is ongoing, these findings indicate an annual growth in the writing and publication of TEs on Scopus. This growth demonstrates that research and publications on TEs are in high demand and receive significant global attention.

Interestingly, 90% of the top 20 source titles contributing to TEs research are journals in the first quartile (Q1) and second quartile (Q2). Among these, 10 journals are in the highest quartile, Q1, and 8 journals are in Q2. The quartile level indicates that these journals have the highest quality and the greatest influence [ 39 , 40 ]. Furthermore, 7 source titles (Philosophical Studies, Synthese, Foundations of Science, Minds and Machines, Erkenntnis, Acta Analytica, and Axiomathes) that publish TEs topics focus on the field of philosophy. When studying TEs from a philosophical standpoint, researchers, scholars, and professionals have the option of submitting their articles to these journals. Alternatively, if TEs are studied from a historical perspective, journals such as Science and Education, Studies in History and Philosophy of Science Part A, Journal for General Philosophy of Science, Philosophy of Science, Boston Studies in the Philosophy and History of Science, and European Journal for Philosophy of Science are appropriate. Meanwhile, if TEs are studied from an educational perspective, Physics Teacher, Science and Education, Physics Education, American Journal of Physics, and European Journal of Physics are ideal choices for publishing articles. These journals regularly publish articles in physics education studies.

If we look at the authors who have made the greatest contributions to the topic of TEs in the previous 20 years (2003–2022), Stuart is the most significant author with 7 articles (51 citations). Stuart’s work focuses on the history and philosophy of TEs [ 31 , 32 , 33 ], with the first publication in 2014 in the journal Perspectives of Science. In contrast to Stuart, Bancong's work, which ranks second, investigates various TEs from an educational standpoint. His first work, published in 2018, examined TEs in high school physics textbooks [ 3 ], followed by an investigation of how students construct TEs collaboratively [ 4 ], and an identification of factors influencing TEs during problem-solving activities [ 5 ]. Following Bancong, Fehige from the University of Toronto, Canada, has also published 5 articles. Most of his work examines TEs in religious contexts, such as thought experiments, Christianity and science in novalis [ 15 ], thought experiments and theology [ 16 ], and the book of job as a thought experiment: on science, religion, and literature [ 17 ] which was published in the journal Religions in 2019. Brown examines TEs in several of his works in light of the history and philosophy of science [ 6 , 7 ], as do Buzzoni and El Skaf from Italy, who mostly discuss TEs in light of the history and philosophy of science [ 8 , 12 ]. Meanwhile, Halkia and Velentzas from the University of Athens, Greece, have discussed TEs from an educational perspective, such as using TEs from Newton's Cannon for teaching satellite physics [ 37 ] and using TEs from the theory of relativity for teaching relativity theories [ 38 ].

Over the past two decades, authors have examined TEs from diverse perspectives, including history, philosophy, education, and religion. This variety highlights a significant shift in the disciplinary landscape of TE research, which is historically rooted in the philosophy of science [ 18 , 24 ]. The true strength of TEs lies in their adaptability across disciplines, rather than in resolving philosophical disputes. Although TEs were traditionally centered on history and philosophy of science (HPS), recent trends show a growing application in education and technology, particularly in artificial intelligence and speculative design. This shift indicates that TEs have not lost their significance but have instead found new areas of relevance. In HPS, the focus has moved toward understanding the methodological and epistemological implications of TEs, confirming their essential role in scientific reasoning [ 7 , 30 ]. Additionally, in fields such as physics education, TEs are increasingly utilized to explore complex theoretical concepts and enhance educational methodologies [ 2 , 12 ].

Based on the most commonly used keywords in the last 20 years, research on TEs has mostly focused on understanding TEs from a philosophical perspective in the first five years (2003–2007). Thought experiments rethought and reperceived [ 19 ], on thought experiments: is there more to the argument? [ 24 ] and thought experiments [ 9 ] are a few examples. Then, over the next five years (2008–2012), many studies looked at how TEs contributed to physical theories, including the special theory of relativity and quantum theory. The keywords that emerged frequently during this period were quantum theory, scientific discovery, methodology, quantum mechanics, twin earths, falling bodies, and others. In the last ten years, TEs have been studied from various perspectives. For example, in 2013, Velentzas and Halkia [ 38 ] also used TEs as a didactic tool in teaching physics to upper-secondary students. Fehige, on the other hand, began to connect TEs to theology, with a specific focus on the interaction between Christianity and science [ 15 , 16 ]. There are also researchers who continue to study the existence of TEs from a philosophical point of view and claim that TEs are science fiction [ 1 , 20 ]. In recent years, TEs have become increasingly popular in education and have been linked to artificial intelligence. Artificial intelligence, physics education, productive imagination, technology, and speculative design are some of the keywords that appear frequently. This is not surprising because TEs, as experimental activities using mental models, are not easy for students to perform on their own [ 4 , 5 ]. Therefore, technology that can assist students in creating an imaginative world for constructing TEs is required.

Since no studies have charted the trends in TEs research so far, it is difficult to compare the research results obtained with those of others. Nevertheless, several studies that examine trends in physics education reveal that although research on experiments is declining in physics education, TEs are still important to physics teaching and learning [ 41 ]. Hallswoth et al. [ 21 ] have also used artificial intelligence technologies to support TEs in the field of wet biology research, which is dominated by experiments on microbial growth and survival. The use of artificial intelligence in learning is based on the growing interest in artificial intelligence methods in science, technology, and education [ 11 ]. Overall, our study contributes to a more comprehensive understanding of TEs research trends during the last 20 years. In addition, this research also contributes to providing an overview of several potential topics that can be the focus of future researchers, such as the use of artificial intelligence in TEs. By situating our findings within the broader context of previous studies, we provide a clearer picture of how TE research has evolved and where it is heading.

5 Conclusions

This study aims to present a more comprehensive understanding of the trend of studies on TEs during the last 20 years (2003–2022). Research on this topic began in 1976, and its progress has continued to the present. A total of 679 published papers from various sources, including articles (504), conference papers (92), and book chapters (83), were analyzed. The results of the study show that research and publications on TEs are of interest and have received a lot of attention during the last eight years. A significant increase occurred in 2021, with 69 published articles. According to the findings, 95% of the top 20 source titles contributing to TEs are from journals in the first and second quartiles (Q1 and Q2). This quartile ranking shows the quality and significant influence of a journal. The geographical distribution reveals that the United States contributes the most to TEs research, with 213 documents, 2592 citations, and 47 links. We also identified several prospective keywords that could be the focus of future research, including artificial intelligence, physics education, fiction, God, theology, productive imagination, technology, speculative design, and critical design. Therefore, this study contributes to providing a thorough picture of thought experiment research trends and future directions of potential topics that can be the focus of future researchers.

This research has several limitations. The exclusive source of publication data utilized in this study is the Scopus database, which is recognized as one of the most extensive databases in the field. However, it is worth noting that future research endeavors may consider including publication data from other prominent sources such as WoS and Google Scholar. Furthermore, the utilization of the search function in the TITLE-ABS-KEY field, specifically employing the terms "Thought-experiments" AND "Science" OR "Physics," was used for the purpose of data retrieval. However, it is important to acknowledge that this approach is not infallible, as there is a potential for some papers to be overlooked, making the process less than completely accurate. Despite its limitations, this research is often regarded as a pioneering contribution to the field of bibliometric studies on the subject of TEs during the past two decades.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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1854, in the meaning defined above

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thought experiment

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A calque of German Gedankenexperiment .

thought experiment ( plural thought experiments )

• ( physics , philosophy ) An attempt to solve a problem using the power of human imagination .

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