Thought experiment

A thought experiment (from the German term Gedankenexperiment, coined by Hans Christian Ørsted) in the broadest sense is the use of an imagined scenario to help us understand the way things really are. The understanding comes through reflection on the situation. Thought experiment methodology is a priori, rather than empirical, in that it does not proceed by observation or experiment.

Thought experiments are well-structured and well-defined (rather than ill-defined) hypothetical questions that employ subjunctive reasoning (irrealis moods). Thought experiments often introduce interesting, important and valuable new perspectives on old mysteries and old questions; yet, although they may make old questions irrelevant, they may also create new questions that may not be easy to answer.

Thought experiments have been used in philosophy, physics, and other fields. They have been used to pose questions in philosophy at least since Greek antiquity, some pre-dating Socrates. In physics and other sciences many famous thought experiments date from the 19th and especially the 20th Century, but examples can be found at least as early as Galileo.

Origins and use of the term "thought experiment"
Witt-Hansen (1986) established that Hans Christian Ørsted was the first to use the Latin-German mixed term "gedankenexperiment" (lit. experiment conducted in the thoughts) circa 1812. Ørsted was also the first to use its entirely German equivalent, "gedankenversuch", in 1820.

Much later, Ernst Mach used the term "gedankenexperiment" to exclusively denote the imaginary conduct of a real experiment that would be subsequently performed as a real physical experiment by his students -- thus the contrast between physical and mental experimentation -- with Mach asking his students to provide him with explanations whenever it happened that the results from their subsequent, real, physical experiment had differed from those of their prior, imaginary experiment.

The English term thought experiment was coined (as a calque) from Mach’s gedankenexperiment, and it first appeared in the 1897 English translation of one of Mach’s papers.

In many ways, the emergence of the term "thought experiment" is a classic case of positioning (see positioning (marketing)). 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). However, people had no way of categorizing it or speaking about it. This helps to explain the extremely wide and diverse range of the application of the term "thought experiment" once it had been introduced into English.

Thought experiments in general
In its broadest usage, thought experimentation is the process of employing imaginary situations to help us understand the way things really are (or, in the case of Herman Kahn’s "scenarios", understand something about something in the future). The understanding comes through reflection upon this imaginary situation. Thought experimentation is an a priori, rather than an empirical process, in that the experiments are conducted within the imagination (i.e., Brown’s (1993) "laboratory of the mind"), and never in fact.

Thought experiments, which are well-structured, well-defined (rather than ill-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 famous thought experiments date from the 19th and especially the 20th Century, but examples can be found at least as early as Galileo.

Thought experiments have been used in philosophy, physics, and other fields (such as cognitive psychology, history, political science, economics, social psychology, law, organizational studies, marketing, and epidemiology).

Scientists tend to use thought experiments in the form of imaginary, "proxy" experiments which they conduct prior to a real, "physical" experiment (Ernst Mach always argued that these gedankenexperiments were "a necessary precondition for physical experiment"). Even today, many scientists argue that these are the only genuine thought experiments. 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 labelled these sorts of experiment "theoretical experiments-in-imagination").

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.

In terms of their function, thought experiments are generally created in order to:
 * challenge (or, even, refute) a prevailing theory,
 * 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.

In terms of their goals, and regardless of their field of application, thought experiments are generally created in order 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;
 * the retrodiction, postdiction and postcasting of the (otherwise) indefinite and unknowable past;
 * facilitate decision making, choice and strategy selection;
 * solve problems, and generate ideas;
 * move current (often insoluble) problems into another, more helpful and more productive problem space (e.g., see 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; or
 * examine the extent to which past events might have occurred differently.
 * ensure the (future) avoidance of past failures.

Seven types of hypothetical question
Generally speaking, the entire domain of thought experiments can be divided into seven types on the basis of the sorts of hypothetical question they ask:

Prefactual thought experiments
Prefactual (“before the fact”) thought experiments speculate on possible future outcomes, given the present, and ask "What will be the outcome if E occurs?"

Counterfactual thought experiments
Counterfactual (“contrary to established fact”) thought experiments speculate on the possible outcomes of a different past; and ask "What might have happened if A had happened instead of B?" (e.g., "If Isaac Newton and Gottfried Leibniz had  cooperated with each other, what would mathematics look like today?").

Semifactual thought experiments
Semifactual thought experiments speculate on the extent to which things might have remained the same, despite there being a different past; and asks the question “Even though X happened instead of E, would Y have still occurred?” (e.g., “Even if the goalie had moved left, rather than right, could he have intercepted a ball that was travelling at such a speed?”).

Semifactual speculations are an important part of clinical medicine.

Prediction, forecasting and nowcasting
The activities of prediction, forecasting and nowcasting attempt to project the circumstances of the present into the future (the only difference between these identically patterned activities being the distance of their speculated future from the present).

Hindcasting
The activity of hindcasting involves running a forecast model after an event has happened in order to test whether the model’s simulation is valid.

Retrodiction (or postdiction)
The activity of retrodiction (or postdiction) involves moving backwards in time, step-by-step, in as many stages as are considered necessary, from the present into the speculated past, in order to establish the ultimate cause of a specific event (e.g., Reverse engineering and Forensics).

Backcasting
The activity of backcasting involves the establishing the description of a very definite and very specific future situation. It then involves an imaginary moving backwards in time, step-by-step, in as many stages as are considered necessary, from the future to the present, in order to reveal the mechanism through which that particular specified future could be attained from the present.

It is important to recognize that a major difficulty with all types of thought experiment, and particularly with counterfactual thought experiments, is that there are no formally accepted criteria for accurately measuring the risk of either Type I errors (False positive) or Type II errors (False negative) in the choice of a potential causative factor.

Thought experiments in philosophy
In philosophy, a thought experiment typically presents an imagined scenario with the intention of eliciting an intuitive 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 intuitive 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 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. 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 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 oftentimes, philosophers differ in their intuitions about the scenario.

The scenario presented in the thought experiment must be possible in some sense. In many thought experiments, the scenario would be possible according to the laws of nature, or nomologically possible. 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 which 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 impugns its supposed intuitive results.

Other uses of imagined scenarios arguably are thought experiments also. In one use of scenarios, we might imagine persons in a particular situation (maybe ourselves), and ask what they would do. 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 various uses of the state of nature, as by Thomas Hobbes and John Locke may also be considered thought experiments.

The use of thought experiments in philosophy has been criticized by some philosophers, especially in the philosophy of mind. Daniel Dennett has derisively referred to thought experiments as "intuition pumps." One criticism that has been voiced is that some science fiction-type thought experiments are too wild to yield clear intuitions, or that any resulting intuitions could not possibly pertain to the real world. Another criticism is that philosophers have used thought experiments (and other a priori methods) in areas where empirical science should be the primary method of discovery, as for example, with issues about the mind.

Thought experiments in physics
Thought experiments in physics are intended to give us a priori knowledge of the natural world, rather than apriori knowledge of our concepts, as philosophy tries to do.

Physics
Thought experiments are popular in physics and include:
 * Brownian ratchet (Richard Feynman's "perpetual motion" machine which does not violate the second law, and does not work)
 * Casimir cones (Basis for almost perpetual motion machine fueled by entropy)
 * Galileo's ship (classical relativity principle) 1632
 * GHZ experiment (quantum mechanics)
 * EPR paradox (quantum mechanics) (forms of this have actually been performed)
 * Maxwell's demon (thermodynamics) 1871
 * Quantum suicide (quantum mechanics)
 * Schrödinger's cat (quantum mechanics)
 * Twin paradox (special relativity)
 * Wigner's friend (quantum mechanics)
 * Wittgenstein's rod (engineering mechanics)- an exercise in visualization
 * Bucket argument- argues that space is absolute, not relational

Philosophy
The field of philosophy makes extensive use of thought experiments:
 * Brain-in-a-vat (epistemology)
 * Changing places (reflexive monism, philosophy of mind)
 * China brain (physicalism, philosophy of mind)
 * Chinese room (philosophy of mind, artificial intelligence, cognitive science)
 * Coherence (philosophical gambling strategy)
 * Dining Philosophers (computer science)
 * God's Debris (religion and awareness)
 * Hilary Putnam's Twin Earth thought experiment in the philosophy of language
 * Mary's room (philosophy of mind)
 * Original position (politics)
 * Philosophical zombie (philosophy of mind, artificial intelligence, cognitive science)
 * Social contract theories
 * The Ship of Theseus (concept of identity)
 * Simulated reality (philosophy, computer science, cognitive science)
 * Swamp man (personal identity)
 * Trolley problem (ethics)
 * The Violinist (ethics)
 * Zeno's paradoxes (classical Greek problems of the infinite)

Mathematics

 * Ping-pong ball conundrum (infinity and cardinality)
 * Gabriel's Horn ('You can fill my horn, but not paint it')

Miscellaneous

 * Braitenberg vehicles (robotics, neural control and sensing systems) (some have actually been built)
 * Doomsday argument (anthropic principle)
 * Infinite monkey theorem (probability, infinity)
 * Halting problem (limits of computability)
 * Turing machine (limits of computability)

Significant articles about thought experiments or thought experimentation

 * Galton, F., "Statistics of Mental Imagery", Mind, Vol.5, No.19, (July 1880), pp.301-318.
 * 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.
 * Mach, E., "On Thought Experiments", pp.134-147 in Mach, E., Knowledge and Error: Sketches on the Psychology of Enquiry, D. Reidel Publishing Co., (Dordrecht), 1976. [Translation of Erkenntnis und Irrtum (5th edition, 1926.].
 * Popper, K., "On the Use and Misuse of Imaginary Experiments, Especially in Quantum Theory", pp.442-456, in Popper, K., The Logic of Scientific Discovery, Harper Torchbooks, (New York), 1968.
 * Witt-Hansen, J., "H.C. Örsted, Immanuel Kant and the Thought Experiment", Danish Yearbook of Philosophy, Vol.13, (1996), pp.48-65.

Books about thought experiments

 * Brown, J.R., The Laboratory of the Mind: Thought Experiments in the Natural Sciences, Routledge, (London), 1993.
 * Browning, K.A. (ed.), Nowcasting, Academic Press, (London), 1982.
 * Craik, K.J.W., The Nature of Explanation, Cambridge University Press, (Cambridge), 1943.
 * Cushing, J.T., Philosophical Concepts in Physics: The Historical Relation Between Philosophy and Scientific Theories, Cambridge University Press, (Cambridge), 1998.
 * DePaul, M. & Ramsey, W. (eds.), Rethinking Intuition: The Psychology of Intuition and Its Role in Philosophical Inquiry, Rowman & Littlefield Publishers, (Lanham), 1998.
 * Gendler, T.S., Thought Experiment: On the Powers and Limits of Imaginary Cases, Garland, (New York), 2000.
 * Gendler, T.S. & Hawthorne, J., Conceivability and Possibility, Oxford University Press, (Oxford), 2002.
 * Häggqvist, S., Thought Experiments in Philosophy, Almqvist & Wiksell International, (Stockholm), 1996.
 * Hanson, N.R., Patterns of Discovery: An Inquiry into the Conceptual Foundations of Science, Cambridge University Press, (Cambridge), 1962.
 * Harper, W.L., Stalnaker, R. & Pearce, G. (eds.), Ifs: Conditionals, Belief, Decision, Chance, and Time, D. Reidel Publishing Co., (Dordrecht), 1981.
 * Hesse, M.B., Models and Analogies in Science, Sheed and Ward, (London), 1963.
 * Holyoak, K.J. & Thagard, P., Mental Leaps: Analogy in Creative Thought, A Bradford Book, The MIT Press, (Cambridge), 1995.
 * Horowitz, T. & Massey, G.J. (eds.), Thought Experiments in Science and Philosophy, Rowman & Littlefield, (Savage), 1991.
 * Kahn, H., Thinking About the Unthinkable, Discus Books, (New York), 1971.
 * Leatherdale, W.H., The Role of Analogy, Model and Metaphor in Science, North-Holland Publishing Company, (Amsterdam), 1974.
 * Roese, N.J. & Olson, J.M. (eds.), What Might Have Been: The Social Psychology of Counterfactual Thinking, Lawrence Erlbaum Associates, (Mahwah), 1995.
 * 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.
 * Sorensen, R.A., Thought Experiments, Oxford University Press, (Oxford), 1992.
 * 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.
 * Vosniadou, S. & Ortony. A. (eds.), Similarity and Analogical Reasoning, Cambridge University Press, (Cambridge), 1989.
 * Wilkes, K.V., Real People: Personal Identity without Thought Experiments, Oxford University Press, (Oxford), 1988.