Thought experiment

Uses

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.

Examples of thought experiments include Schrödinger's cat, that was meant to attack the Copenhagen Interpretation of quantum mechanics by showing that its assumptions could lead to the seemingly absurd condition of a cat being simultaneously alive and dead, and Maxwell's demon, which attempts to demonstrate the ability of a hypothetical finite being to violate the Second Law of Thermodynamics.

It is a common element of science-fiction stories.

Thought experiments, which are well-structured, well-defined hypothetical questions that employ subjunctive 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-dating Socrates. 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 as Galileo.

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.

Thought experiments have been used in philosophy (especially ethics), physics, and other fields (such as cognitive psychology, history, political science, economics, social psychology, law, organizational studies, marketing, and epidemiology). In law, the synonym "hypothetical" is frequently used for such experiments.

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.

Theoretical consequences

In terms of their theoretical consequences, thought experiments generally:

• challenge (or even refute) a prevailing theory, often involving the device known as reductio ad absurdum, (as in Galileo's original argument, a proof by contradiction),
• confirm a prevailing theory,
• establish a new theory, or
• simultaneously refute a prevailing theory and establish a new theory through a process of mutual exclusion

Practical applications

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 correcting misinformation (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)
• extrapolate beyond (or interpolate within) the boundaries of already established fact
• predict and forecast the (otherwise) indefinite and unknowable future
• explain the past
• facilitate the retrodiction, postdiction and hindcasting of the otherwise indefinite and unknowable past
• facilitate decision making, choice, and strategy selection
• solve problems, and generate ideas;
• move current unsolved problems into another more productive problem space (e.g. functional fixedness)
• attribute causation, preventability, blame, and responsibility for specific outcomes
• assess culpability and compensatory damages in social and legal contexts
• ensure the repeat of past success
• examine the extent to which past events might have occurred differently
• ensure the future avoidance of past failures

Fields

Thought experiments have been used in a variety of fields, including philosophy, law, physics, and mathematics. In philosophy they have been used at least since classical antiquity, some pre-dating Socrates. In law, they were well known to Roman lawyers quoted in the Digest. 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 as Galileo.

Philosophy

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 (See Consequentialism). 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 a functionalist 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.

For example, in the veil of ignorance, John Rawls asks us to imagine a group of persons in a situation where they know nothing about themselves, and are charged with devising a social or political organization. The use of the state of nature to imagine the origins of government, as by Thomas Hobbes and John Locke, may also be considered a thought experiment. Søren Kierkegaard explored the possible ethical and religious implications of Abraham's binding of Isaac in Fear and Trembling. Similarly, Friedrich Nietzsche, in On the Genealogy of Morals, speculated about the historical development of Judeo-Christian morality, with the intent of questioning its legitimacy.

An early written thought experiment was Plato's allegory of the cave. Another historic thought experiment was Avicenna's "floating man" thought experiment in the 11th century. He asked his readers to imagine themselves suspended in the air isolated from all sensations in order to demonstrate human self-awareness and self-consciousness, and the substantiality of the soul.

Science

Scientists tend to use thought experiments as imaginary, "proxy" experiments prior to a real, "physical" experiment (Ernst Mach always argued that these gedankenexperiments 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 as Einstein's thought experiment of chasing a light beam, leading to special 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.

Properties

Further categorization of thought experiments can be attributed to specific properties.

Possibility

In many thought experiments, the scenario would be nomologically possible, or possible according to the laws of nature. John Searle's Chinese room is nomologically possible.

Some thought experiments present scenarios that are not nomologically possible. In his Twin 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 as metaphysical 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 are zombies, or persons who are physically identical to us in every way but who lack consciousness. This is supposed to show that physicalism 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.

Causal reasoning

The first characteristic pattern that thought experiments display is their orientation in time. 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 present moment standpoint" of the individual performing the experiment; namely, in terms of:

• Their temporal direction: are they past-oriented or future-oriented?
• Their temporal 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?

Relation to real experiments

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, on the basis of a rigorous analysis of a certain, complicated, but in the meantime assertedly realizable model, he came to the conclusion that quantum mechanics should be described as "incomplete". Niels Bohr asserted a refutation of Einstein's analysis immediately, and his view prevailed. After some decades, it was asserted that feasible experiments could prove the error of the EPR paper. These experiments tested the Bell 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 optical real experiments of Alain Aspect).

Thus thought experiments belong to a theoretical discipline, usually to theoretical physics, but often to theoretical philosophy. In any case, it must be distinguished from a real experiment, which belongs naturally to the experimental discipline and has "the final decision on true or not true", at least in physics.

Interactivity

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 of Stefano Gualeni, for example, focuses on the use of virtual worlds to materialize thought experiments and to playfully negotiate philosophical ideas. His arguments were originally presented in his 2015 book Virtual Worlds as Philosophical Tools.

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. 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.

Examples

Humanities

• Doomsday argument (anthropic principle)
• The Lady, or the Tiger? (human nature)
• The beer question (U.S. politics)

Physics

• Bell's spaceship paradox (special relativity)
• Brownian ratchet (Richard Feynman's "perpetual motion" machine that does not violate the second law and does no work at thermal equilibrium)
• Bucket argument – argues that space is absolute, not relational
• Dyson sphere
• Einstein's box
• Elitzur–Vaidman bomb-tester (quantum mechanics)
• EPR paradox (quantum mechanics) (forms of this have been performed)
• Everett phone (quantum mechanics)
• Feynman sprinkler (classical mechanics)
• Galileo's Leaning Tower of Pisa experiment (rebuttal of Aristotelian Gravity)
• Galileo's ship (classical relativity principle) 1632
• GHZ experiment (quantum mechanics)
• Heisenberg's microscope (quantum mechanics)
• Kepler's Dream (change of point of view as support for the Copernican hypothesis)
• Ladder paradox (special relativity)
• Laplace's demon
• Maxwell's demon (thermodynamics) 1871
• Mermin's device (quantum mechanics)
• Moving magnet and conductor problem
• Newton's cannonball (Newton's laws of motion)
• Popper's experiment (quantum mechanics)
• Quantum pseudo telepathy (quantum mechanics)
• Quantum suicide and immortality (quantum mechanics)
• Renninger negative-result experiment (quantum mechanics)
• Schrödinger's cat (quantum mechanics)
• Sticky bead argument (general relativity)
• The Monkey and the Hunter (gravitation)
• Twin paradox (special relativity)
• Wheeler's delayed choice experiment (quantum mechanics)
• Wigner's friend (quantum mechanics)

Philosophy

• Artificial brain
• Avicenna's Floating man
• Beetle in a box
• Bellum omnium contra omnes
• Big Book (ethics)
• Brain-in-a-vat (epistemology, philosophy of mind)
• Brainstorm machine
• Buridan's ass
• Changing places (reflexive monism, philosophy of mind)
• Chesterton's fence
• China brain (physicalism, philosophy of mind)
• Chinese room (philosophy of mind, artificial intelligence, cognitive science)
• Coherence (philosophical gambling strategy)
• Condillac's Statue (epistemology)
• Experience machine (ethics)
• Gettier problem (epistemology)
• Ḥayy ibn Yaqẓān (epistemology)
• Hilary Putnam's Twin Earth thought experiment in the philosophy of language and philosophy of mind
• If a tree falls in a forest
• Inverted spectrum
• Kavka's toxin puzzle
• Mary's room (philosophy of mind)
• Molyneux's Problem (admittedly, this oscillated between empirical and a-priori assessment)
• Newcomb's paradox
• Original position (politics)
• Philosophical zombie (philosophy of mind, artificial intelligence, cognitive science)
• Plank of Carneades
• Roko's basilisk
• Ship of Theseus, The (concept of identity)
• Shoemaker's "Time Without Change" (metaphysics)
• Simulated reality (philosophy, computer science, cognitive science)
• Social contract theories
• Survival lottery (ethics)
• Swamp man (personal identity, philosophy of mind)
• Teleportation (metaphysics)
• The transparent eyeball
• The violinist (ethics)
• Ticking time bomb scenario (ethics)
• Trolley problem (ethics)
• Utility monster (ethics)
• Zeno's paradoxes (classical Greek problems of the infinite)

Mathematics

• Balls and vase problem (infinity and cardinality)
• Gabriel's Horn (infinity)
• Hilbert's paradox of the Grand Hotel (infinity)
• Infinite monkey theorem (probability)
• Lottery paradox (probability)
• Sleeping beauty paradox (probability)

Biology

• Levinthal paradox
• Rotating locomotion in living systems

Computer science

• Braitenberg vehicles (robotics, neural control and sensing systems)
• Dining Philosophers
• Two Generals' Problem

Economics

• Broken window fallacy (law of unintended consequences, opportunity cost)
• Laffer Curve