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Athought experiment is an imaginaryscenario that is meant to elucidate or test an argument ortheory. It is often an experiment that would be hard, impossible, orunethical to actually perform. It can also be an abstracthypothetical that is meant to test our intuitions about morality or other fundamental philosophical questions.[2][3][4][5][6]
The ancient Greekδείκνυμι,deiknymi,'thought experiment', "was the most ancient pattern ofmathematical proof", and existed beforeEuclidean mathematics,[7] where the emphasis was on the conceptual, rather than on the experimental part of a thought experiment.
Johann Witt-Hansen established thatHans Christian Ørsted was the first to use the equivalent German termGedankenexperimentc. 1812.[8][9] Ørsted was also the first to use the equivalent termGedankenversuch in 1820.
By 1883,Ernst Mach usedGedankenexperiment in a different sense, to denote exclusively theimaginary conduct of areal experiment that would be subsequently performed as areal physical experiment by his students.[10] Physical and mental experimentation could then be contrasted: Mach asked his students to provide him with explanations whenever the results from their subsequent, real, physical experiment differed from those of their prior, imaginary experiment.
The English termthought experiment was coined as acalque ofGedankenexperiment, and it first appeared in the 1897 English translation of one of Mach's papers.[11] Prior to its emergence, the activity of posing hypothetical questions that employed subjunctive reasoning had existed for a very long time for both scientists and philosophers. Theirrealis moods are ways to categorize it or to speak about it. This helps explain the extremely wide and diverse range of the application of the termthought experiment once it had been introduced into English.
Galileo's thought experiment concerned the outcome (c) of attaching a small stone (a) to a larger one (b).
Galileo's demonstration that falling objects must fall at the same rate regardless of their masses was a significant step forward in the history of modern science. This is widely thought[12] to have been a straightforward physical demonstration, involving climbing up the Leaning Tower of Pisa and dropping two heavy weights off it, whereas in fact, it was a logical demonstration, using the thought experiment technique. The experiment is described by Galileo in his 1638 workTwo New Sciences thus:
Salviati:If then we take two bodies whose natural speeds are different, it is clear that on uniting the two, the more rapid one will be partly retarded by the slower, and the slower will be somewhat hastened by the swifter. Do you not agree with me in this opinion? Simplicio:You are unquestionably right. Salviati:But if this is true, and if a large stone moves with a speed of, say, eight while a smaller moves with a speed of four, then when they are united, the system will move with a speed less than eight; but the two stones when tied together make a stone larger than that which before moved with a speed of eight. Hence the heavier body moves with less speed than the lighter; an effect which is contrary to your supposition. Thus you see how, from your assumption that the heavier body moves more rapidly than the lighter one, I infer that the heavier body moves more slowly.[13]
Thought experiments may be used to explore a hypothesis and the implementation of theories around it. They are also used in education, or for personal entertainment.[9]
Examples of thought experiments includeSchrödinger's cat, that was meant to attack theCopenhagen Interpretation of quantum mechanics by showing that its assumptions could lead to the seemingly absurd condition of a cat being simultaneously alive and dead, andMaxwell's demon, which attempts to demonstrate the ability of a hypothetical finite being to violate the2nd law of thermodynamics.
Thought experiments, which are well-structured, well-defined hypothetical questions that employsubjunctive reasoning (irrealis moods) – "What might happen (or, what might have happened) if . . . " – have been used to pose questions in philosophy at least since Greek antiquity, some pre-datingSocrates.[15] In physics and other sciences many thought experiments date from the 19th and especially the 20th Century, but examples can be found at least as early asGalileo.
In thought experiments, we gain new information by rearranging or reorganizing empirical data in a new way and drawing new inferences from them, or by looking at these data from a different and unusual perspective. In Galileo's thought experiment, for example, the rearrangement of empirical experience consists of the original idea of combining bodies of different weights.[16]
Regardless of their intended goal, all thought experiments display a patterned way of thinking that is designed to allow us to explain, predict, and control events in a better and more productive way.
Thought experiments can produce some very important and different outlooks on previously unknown or unaccepted theories. However, they may make those theories themselves irrelevant, and could possibly create new problems that are just as difficult, or possibly more difficult to resolve.
In terms of their practical application, thought experiments are generally created to:
challenge the prevailing status quo (which includes activities such as correctingmisinformation (or misapprehension), identify flaws in the argument(s) presented, to preserve (for the long-term) objectively established fact, and to refute specific assertions that some particular thing is permissible, forbidden, known, believed, possible, or necessary)
Thought experiments have been used in a variety of fields, including philosophy, law,physics, and mathematics. In philosophy they have been used at least sinceclassical antiquity, some pre-datingSocrates. In law, they were well known toRoman lawyers quoted in theDigest.[17] In physics and other sciences, notable thought experiments date from the 19th and, especially, the 20th century; but examples can be found at least as early asGalileo.
In philosophy, a thought experiment typically presents an imagined scenario with the intention of eliciting an intuitive or reasoned response about the way things are in the thought experiment. (Philosophers might also supplement their thought experiments with theoretical reasoning designed to support the desired intuitive response.) The scenario will typically be designed to target a particular philosophical notion, such as morality, or the nature of the mind or linguistic reference. The response to the imagined scenario is supposed to tell us about the nature of that notion in any scenario, real or imagined.
For example, a thought experiment might present a situation in which an agent intentionally kills an innocent for the benefit of others. Here, the relevant question is not whether the action is moral or not, but more broadly whether a moral theory is correct that says morality is determined solely by an action's consequences (SeeConsequentialism).John Searle imagines a man in a locked room who receives written sentences in Chinese, and returns written sentences in Chinese, according to a sophisticated instruction manual. Here, the relevant question is not whether or not the man understands Chinese, but more broadly, whether afunctionalist theory of mind is correct.
It is generally hoped that there is universal agreement about the intuitions that a thought experiment elicits. (Hence, in assessing their own thought experiments, philosophers may appeal to "what we should say," or some such locution.) A successful thought experiment will be one in which intuitions about it are widely shared. But often, philosophers differ in their intuitions about the scenario.
Other philosophical uses of imagined scenarios arguably are thought experiments also. In one use of scenarios, philosophers might imagine persons in a particular situation (maybe ourselves), and ask what they would do.
Scientists tend to use thought experiments as imaginary, "proxy" experiments prior to a real, "physical" experiment (Ernst Mach always argued that thesegedankenexperiments were "a necessary precondition for physical experiment"). In these cases, the result of the "proxy" experiment will often be so clear that there will be no need to conduct a physical experiment at all.
Scientists also use thought experiments when particular physical experiments are impossible to conduct (Carl Gustav Hempel labeled these sorts of experiment "theoretical experiments-in-imagination"), such asEinstein's thought experiment of chasing a light beam, leading tospecial relativity. This is a unique use of a scientific thought experiment, in that it was never carried out, but led to a successful theory, proven by other empirical means.
Some thought experiments present scenarios that are not nomologically possible. In hisTwin Earth thought experiment,Hilary Putnam asks us to imagine a scenario in which there is a substance with all of the observable properties of water (e.g., taste, color, boiling point), but is chemically different from water. It has been argued that this thought experiment is not nomologically possible, although it may be possible in some other sense, such asmetaphysical possibility. It is debatable whether the nomological impossibility of a thought experiment renders intuitions about it moot.
In some cases, the hypothetical scenario might be considered metaphysically impossible, or impossible in any sense at all.David Chalmers says that we can imagine that there arezombies, or persons who are physically identical to us in every way but who lack consciousness. This is supposed to show thatphysicalism is false. However, some argue that zombies are inconceivable: we can no more imagine a zombie than we can imagine that 1+1=3. Others have claimed that the conceivability of a scenario may not entail its possibility.
The first characteristic pattern that thought experiments display is their orientationin time.[20] They are either:
Antefactual speculations: experiments that speculate about what might have happened prior to a specific, designated event, or
Postfactual speculations: experiments that speculate about what may happen subsequent to (or consequent upon) a specific, designated event.
The second characteristic pattern is their movement in time in relation to "the presentmoment standpoint" of the individual performing the experiment; namely, in terms of:
Theirtemporal direction: are they past-oriented or future-oriented?
Theirtemporal sense:
(a) in the case of past-oriented thought experiments, are they examining the consequences of temporal "movement" from the present to the past, or from the past to the present? or,
(b) in the case of future-oriented thought experiments, are they examining the consequences of temporal "movement" from the present to the future, or from the future to the present?
The relation to real experiments can be quite complex, as can be seen again from an example going back to Albert Einstein. In 1935, with two coworkers, he published a paper on a newly created subject called later the EPR effect (EPR paradox). In this paper, starting from certain philosophical assumptions,[21] on the basis of a rigorous analysis of a certain, complicated, but in the meantime assertedly realizable model, he came to the conclusion thatquantum mechanics should be described as "incomplete".Niels Bohr asserted a refutation of Einstein's analysis immediately, and his view prevailed.[22][23][24] After some decades, it was asserted that feasible experiments could prove the error of the EPR paper. These experiments tested theBell inequalities published in 1964 in a purely theoretical paper. The above-mentioned EPR philosophical starting assumptions were considered to be falsified by the empirical fact (e.g. by the opticalreal experiments ofAlain Aspect).
Thusthought experiments belong to a theoretical discipline, usually totheoretical physics, but often totheoretical philosophy. In any case, it must be distinguished from a real experiment, which belongs naturally to the experimental discipline and has "the final decision ontrue ornot true", at least in physics.
Thought experiments can also be interactive where the author invites people into his thought process through providing alternative paths with alternative outcomes within the narrative, or through interaction with a programmed machine, like a computer program.
Thanks to the advent of the Internet, the digital space has lent itself as a new medium for a new kind of thought experiments. The philosophical work ofStefano Gualeni, for example, focuses on the use of virtual worlds to materialize thought experiments and to playfully negotiate philosophical ideas.[25] His arguments were originally presented in his 2015 bookVirtual Worlds as Philosophical Tools.[26]
Gualeni's argument is that the history of philosophy has, until recently, merely been the history of written thought, and digital media can complement and enrich the limited and almost exclusively linguistic approach to philosophical thought.[26][25][27] He considers virtual worlds (like those interactively encountered in videogames) to be philosophically viable and advantageous. This is especially the case in thought experiments, when the recipients of a certain philosophical notion or perspective are expected to objectively test and evaluate different possible courses of action, or in cases where they are confronted with interrogatives concerning non-actual or non-human phenomenologies.[26][25][27]
^Witt-Hansen (1976). AlthoughExperiment is a German word, it is derived fromLatin. The synonymVersuch has purelyGermanic roots.
^abBrown, James Robert; Fehige, Yiftach (30 September 2019) [1996]."Thought Experiments". In Zalta, Edward N. (ed.).The Stanford Encyclopedia of Philosophy. Metaphysics Research Lab, Stanford University.
^Mach, Ernst (1883), The Science of Mechanics (6th edition, translated by Thomas J. McCormack), LaSalle, Illinois: Open Court, 1960. pp. 32–41, 159–62.
^Mach, Ernst (1897), "On Thought Experiments", inKnowledge and Error (translated by Thomas J. McCormack and Paul Foulkes), Dordrecht Holland: Reidel, 1976, pp. 134-47.
^Cohen, Martin, "Wittgenstein's Beetle and Other Classic Thought Experiments", Blackwell, (Oxford), 2005, pp. 55–56.
^Jaynes, E.T. (1989).Clearing up the Mysteries, opening talk at the 8th International MAXENT Workshop, St John's College, Cambridge UK.
^French, A.P., Taylor, E.F. (1979/1989).An Introduction to Quantum Physics, Van Nostrand Reinhold (International), London,ISBN0-442-30770-5.
^Wheeler, J.A, Zurek, W.H., editors (1983).Quantum Theory and Measurement, Princeton University Press, Princeton.
^d'Espagnat, B. (2006).On Physics and Philosophy, Princeton University Press, Princeton,ISBN978-0-691-11964-9
^abcGualeni, Stefano (21 April 2022)."Philosophical Games".Encyclopedia of Ludic Terms. Retrieved6 August 2024.
^abcGualeni, Stefano (2015).Virtual Worlds as Philosophical Tools: How to Philosophize with a Digital Hammer. Basingstoke (UK): Palgrave MacMillan.ISBN978-1-137-52178-1.
^While the problem presented in this short story's scenario is not unique, it is extremely unusual. Most thought experiments are intentionally (or, even, sometimes unintentionally) skewed towards the inevitable production of a particular solution to the problem posed; and this happens because of the way that the problem and the scenario are framed in the first place. In the case ofThe Lady, or the Tiger?, the way that the story unfolds is so "end-neutral" that, at the finish, there is no "correct" solution to the problem. Therefore, all that one can do is to offer one's own innermost thoughts on how the account of human nature that has been presented might unfold – according to one's own experience of human nature – which is, obviously, the purpose of the entire exercise. The extent to which the story can provoke such an extremely wide range of (otherwiseequipollent) predictions of the participants' subsequent behaviour is one of the reasons the story has been so popular over time.
Dennett, D.C., "Intuition Pumps", pp. 180–197 in Brockman, J.,The Third Culture: Beyond the Scientific Revolution, Simon & Schuster, (New York), 1995.ISBN978-0-684-80359-3
Hempel, C.G., "Typological Methods in the Natural and Social Sciences", pp. 155–171 in Hempel, C.G. (ed.),Aspects of Scientific Explanation and Other Essays in the Philosophy of Science, The Free Press, (New York), 1965.
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Ørsted, Hans Christian (1997).Selected Scientific Works of Hans Christian Ørsted. Princeton.ISBN978-0-691-04334-0.. Translated to English by Karen Jelved, Andrew D. Jackson, and Ole Knudsen, (translators 1997).
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Shanks, N. (ed.),Idealization IX: Idealization in Contemporary Physics (Poznan Studies in the Philosophy of the Sciences and the Humanities, Volume 63), Rodopi, (Amsterdam), 1998.
Shick, T. & Vaugn, L.,Doing Philosophy: An Introduction through Thought Experiments (Second Edition), McGraw Hill, (New York), 2003.
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Tetlock, P.E. & Belkin, A. (eds.),Counterfactual Thought Experiments in World Politics, Princeton University Press, (Princeton), 1996.
Thomson, J.J. {Parent, W. (ed.)},Rights, Restitution, and Risks: Essays in Moral Theory, Harvard University Press, (Cambridge), 1986.
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