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9 Philosophical Thought Experiments You Should Know About

Thought experiments are imaginative devices that can help us better understand philosophy. They are a useful tool in education and entertainment and can be a great way to apply complex concepts to practical situations. Since philosophy looks at questions about life, you can make a thought experiment for nearly any philosophical idea. Learn about some of the most popular ones commonly used in philosophical discussions.

The Trolly Problem

The trolly problem is an ethical thought experiment. It first appeared in philosopher Philippa Foot's 1967 paper, "Abortion and the Doctrine of Double Effect." To start off the thought experiment, imagine you have control of a railway switch. There is an out-of-control trolley headed your way. Up ahead, the tracks of the railway branch into two different paths. On the first track, there is a group of five people; on the other track, there is one person. If you stand, watch, and do nothing, the trolley will head down the first track and kill five people. However, your control allows you to switch the path of the trolley. This way, it heads down track two and kills one person instead of five. The dilemma in this situation is whether or not to flip the switch. A utilitarian answer would be to flip the switch and kill one instead of five.

Selective Surgery

Imagine you are a doctor in the future, and an ill patient comes into your practice. The patient's symptoms lead you to the diagnosis that their heart is failing. Without treatment, the patient is going to die. In your office, the patient passes out. Fortunately, the patient can be saved with surgery that will give them a synthetic heart. The patient can live what you consider to be a good life after this surgery. However, as you are preparing the patient for surgery, a small card falls out of their pocket. The card says for religious reasons; the patient does not want any synthetic organs. Now you must make a decision. If you do not install the heart, the patient will die. However, if you install the heart, the patient will survive at the cost of you violating their wish to have no synthetic organs.

At the heart of this thought experiment is a choice between honoring someone's individual rights and honoring an outside moral code . This is a relevant topic today in bioethics. Philosophers who advocate strongly for personal rights would argue that doing the transplant is wrong. While philosopher John Locke wasn't exposed to this thought experiment, he is someone who expressed the importance of individual rights and consent. According to Locke, individual consent is a fundamental part of creating a political society. In his view, doing the transplant on an unconsenting individual would be wrong.

The Bad Father

A thought experiment that tests loyalty against ethical principles is the bad father. In the thought experiment, there is a son who holds honesty as the highest value. However, his father is not an honest man. One day, his son catches him stealing from a local farmer. In this situation, the son must make a choice. He could turn his father in for breaking the law and stealing. Or the son might feel an ethical obligation to keep silent about his father's crime. While you might find this question silly, ask yourself the implications this scenario has on a larger scale. Would it be better for children to stay loyal and protect their parents or better for them to alert the authorities when their parents stray from the law?

This thought experiment is a variation of the thought experiments proposed by the ancient Greek philosopher Plato. He wrote a dialogue called Euthyphro, where Euthyphro takes his father, Socrates, to court. Euthypro argues taking his father to court is the right thing to do and the pious thing to do. However, Socrates disagrees with Euthypro about his impiety charges and gets into a debate with Euthypro about philosophical questions of universal justice, goodness, and piety. At the core of the philosophical thought experiment is the question of what happens when there is a conflict between our personal lives and the impersonal tenets we believe justice demands. The thought experiment brings old philosophical questions about justice to life.

Prisoners Dilemma

The prisoner's dilemma is a game that was created as a model of human cooperation. The experiment shows how people choose to cooperate or how they don't. The mathematician Albert Tucker is credited with formalizing the thought experiment. Today, a wide array of disciplines use the thought experiment, including philosophy, psychology, economics, and political theory.

For the experiment, imagine a cop arrests both Chris and Cindy for robbing a bank. They are in separate cells where they cannot communicate. Both want to be free and care more about their personal freedom than the freedom of their accomplice. A clever prosecutor will use their desire for freedom to his or her advantage. The prosecutor will tell each person separately that if they confess and their accomplice remains silent, they will drop all charges against them and their testimony to ensure their accomplice does serious time. If they both confess, the prosecutor will ensure they both get early parole. If they both remain silent, they will have to settle for sentences on firearms possession charges. The dilemma for the prisoners is that if they both confess, the outcome is worse than it would have been if they had both remained silent.

The prisoner's dilemma compares individual and group rationality. It shows there can be conflict between individual and collective interests. Conclusions drawn from the Prisoner's Dilemma have been used in modern-day philosophical discussions about arms races and the use of limited natural resources.

The Chinese Room

A thought experiment about artificial intelligence is the Chinese room, designed by philosopher John Searle. He asks us to imagine a situation where someone who only knows the English language is sitting alone in a room. They have instructions for changing rows of Chinese letters. Anyone outside the room would think the person inside the room understood Chinese since they would see them sorting through Chinese characters. However, this is not the case. The person inside the room just understands the instructions. Searle made this thought experiment to show that artificial intelligence cannot have a human-like mind. His thought experiment stresses that while there is understanding, there is not comprehension. 

The Experience Machine

A thought experiment that will make you question the value behind experience, is Robert Nozick's Experience Machine. The experiment is from his book Anarchy State and Utopia . Imagine that there is a machine that will give you any experience you desire. You could enter the machine and have an experience that you were eating the world's best cookie or that you were an astronaut. While you are in the experience machine, you are floating in a tank with electrodes attached to your brain. The question here is if you should plug into this machine to preprogram your life experiences. While in the tank, you wouldn't know that you were in the tank, making the experience even more real. The thought experiment brings questions about the meaning of life. What is the purpose of life if we are plugging into a machine? Will plugging into a machine satisfy all of our desires?

The Ship of Theseus

The Ship of Theseus is a thought experiment that questions whether an object that has all of its components replaced or rearranged is in fact the original object. This paradox was recorded by Plutarch, Theseus, who asks if a ship that was fully restored and replaced completely, down to every single wooden part, was the same ship. Later, other philosophers expanded on this idea. Thomas Hobbes asked if the original planks of the first ship were entirely replaced and then the original planks were used to build another ship, if the second ship would be the original ship. The thought experiment asks questions about what the essence of an object is. The Greek philosopher Heraclitus attempted to solve this paradox. To do this, Heraclitus thought of a river that has water replenishing it. According to Heraclitus, this is the same river. However, Plutarch disagreed and claimed the nature of a river to scatter and then come together means you never step into the same river twice.

Original Position

Ever thought the system was unfair? The original position is a thought experiment centering around achieving a better form of justice. Developed by John Rawls, the thought experiment asks us to imagine that we are in a situation where we do not know our actual life. This way, we are behind what Rawls calls a veil of ignorance. This veil prevents us from knowing the political or economic system that we live under and the laws that are in place.  From this position, we are then asked, with a group of other people behind the veil of ignorance, to look at a list of classic forms of justice. We must draw conclusions from different social and political philosophies. Then, we have to choose a system of justice that we believe would be best for everyone under this veil. This thought experiment calls us to question our beliefs about justice. It forces us to confront the flaws of our political and economic systems.

The Beetle in the Box

The Beetle in The Box thought experiment is also known as the Private Language Argument. Philosopher Ludwig Wittgenstein developed the thought experiment to challenge the way we look at introspection and how it informs the language we use. The thought experiment starts by imagining a group of individuals, each holding a box. The boxes contain what each individual calls their beetle. Nobody can see into anyone else's box. Everyone describes their beetle to each other. However, each person only sees and knows their own beetle. According to Wittgenstein, the descriptions are unimportant. This is because, over time, the individuals would understand the word beetle as the thing in a person's box. While the thought experiment might sound silly, it makes the comparison that human minds are like a beetle. We can never know what is in another individual's mind.

Why Use Thought Experiments?

Thought experiments help us explore philosophical concepts in a more practical way. For example, the trolly problem forces us to confront how we would apply our ethics in a situation. The point of thought experiments isn't to arrive at a specific answer. Instead, thought experiments force us to reason through our ideas and give us insight into solving complex questions. When you come up with an answer to a thought experiment, why you arrived at your answer is just as important as the answer itself.

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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.]

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What is a Thought Experiment, Anyhow?

Our philosophical science correspondent massimo pigliucci asks..

Philosophers are often accused of engaging in armchair speculation, as far removed from reality as possible, inside the proverbial ivory tower. The quintessential example of this practice is the thought experiment, which many scientists sneer at precisely because it doesn’t require one to get one’s hands dirty. And yet scientists have often engaged in thought experiments, some of which have marked major advances in our understanding of the world.

Just consider the famous example of Galileo’s thought experiment demonstrating (rather counterintuitively) that two objects of different weight must fall at the same speed. (Contrary to popular belief, Galileo never actually climbed the leaning tower of Pisa to do this experiment – he didn’t need to.) Galileo knew Aristotle would have predicted that a heavy body (H) would fall faster than a lighter one (L). But, the Italian scientist reckoned, suppose we connect the two bodies by a string, thereby making the compound object H+L. Following Aristotelian physics, one would predict that H+L should fall faster than H by itself because of the compound weight: therefore H+L > H. However, it’s also possible to use the same logic to claim that the compound body should fall at a slower pace than H because of the drag created by L, so that H+L < H. But this yields a contradiction, which means – by reductio ad absurdum – that really H = L = H+L. Neil Armstrong, the first human to set foot on the moon, dramatically showed the whole world that Galileo was right when he let go of a hammer and a feather in the absence of atmospheric friction while standing on our satellite, and, sure enough, they hit the Moon’s surface at the same time. Such is the predictive power of thought experiments!

Then again, some thought experiments can lead to misleading conclusions – as in the case of Lucretius’ ‘demonstration’ that space must be infinite. In the De Rerum Natura he reasoned that one might walk up to the boundary of the universe and throw a spear (there were no weapons of mass destruction in Roman times). If the spear flies through the alleged boundary, then it is no boundary at all, and we conclude that the universe is infinite. But what if the spear bounces back? Then there is a boundary; but by definition if there is a boundary then there must be something beyond it – which again leads to the conclusion that the universe is infinite. The problem with this is that today’s mathematics and physics show us how a universe can be both finite and unbounded (it’s like a toroid, ie a donut).

These and other thought experiments are discussed in an elegant paper by James Robert Brown, who goes on to ask what sort of beast, exactly, is a thought experiment? Brown contrasts two theories, his own – according to which thought experiments are a perception of a kind of Platonic reality – and that of John Norton, who thinks that thought experiments are actually a form of argument. I will not take sides, partly because I’m not sure that the two views are actually incompatible with each other; but it is instructive to examine both views in an attempt to wrap our minds around what exactly our minds are doing in these cases.

Let’s start with Brown’s position, which he claims to derive from two starting points: a Platonic view of mathematics, and a realist view of the laws of nature. Platonism in mathematics is the idea that certain entities – like numbers, and relations among numbers – are ‘out there’ independently of human minds. Numbers in some sense ‘exist’ regardless of the presence or ability of a mind to conceive them. Accordingly, mathematicians are akin to scientists: they do not invent things, they discover them. Similarly, following Brown, natural laws like say the law of gravity described by Galileo and later formalized by Newton are ‘real’ in the sense that they exist, again, independently of human observers. (If all this talk of human-independent phenomena has you wondering about the sound of a tree falling when there is nobody there to hear it, stop right now:the analogy is only superficial.)

What does all of this have to do with thought experiments? According to Brown, thought experiments are genuine examples of how the human mind can ‘perceive’ laws of nature by simply thinking about reality. This was the goal of rationalist (as opposed to empiricist) philosophers since Plato: to discover things about the world by sheer intellectual power, independently of empirical evidence, which was seen as unreliable.

In contrast, Norton has a very different take on the whole matter. For him, thought experiments are a form of argument, starting from empirically derived premises and reaching conclusions by deductive logic. In essence, Norton thinks of experiments such as Galileo’s as ‘if-then’ forms of reasoning, which yield valid results when the premises are empirically justified and the reasoning is logically correct.

There is something very appealing about both Brown’s and Norton’s notions. On the one hand, with Brown there is a sense in which thought experiments are formalizations of an intuitive grasp of an objective reality. Yet this reality – if it exists – is surely of the Platonic, abstract type, not of the more mundane “this table is real” kind. On the other hand, it is hard to resist Norton’s construction of thought experiments as arguments based on a proper mix of induction (the empirical premises) and deduction.

Whatever thought experiments really are, they have been instrumental in the progress of both philosophy and science, and they constitute a powerful tool for understanding the world. True, sometimes they don’t work, but the same can be said for physical experiments. In both cases it’s all in the soundness of the premises and the rigor with which the conclusions are derived.

© Dr Massimo Pigliucci 2006

Massimo Pigliucci has a PhD in evolutionary biology and one in philosophy. He is a professor at SUNY-Stony Brook on Long Island, New York. His ramblings can be found at www.rationallyspeaking.org .

Further reading: • Brown, J.R. 2004. ‘Peeking into Plato’s heaven.’ Philosophy of Science 71.

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