Thought experiment

Thought experiment

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A thought experiment or Gedankenexperiment (from German) considers some hypothesis
Hypothesis
A hypothesis is a proposed explanation for a phenomenon. The term derives from the Greek, ὑποτιθέναι – hypotithenai meaning "to put under" or "to suppose". For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...

, theory
Theory
The English word theory was derived from a technical term in Ancient Greek philosophy. The word theoria, , meant "a looking at, viewing, beholding", and referring to contemplation or speculation, as opposed to action...

, or principle
Principle
A principle is a law or rule that has to be, or usually is to be followed, or can be desirably followed, or is an inevitable consequence of something, such as the laws observed in nature or the way that a system is constructed...

 for the purpose of thinking through its consequences. Given the structure of the experiment, it may or may not be possible to actually perform it, and, in the case that it is possible for it to be performed, there need be no intention of any kind to actually perform the experiment in question. The common goal of a thought experiment is to explore the potential consequences of the principle in question.

Famous examples of thought experiments include Schrödinger's cat
Schrödinger's cat
Schrödinger's cat is a thought experiment, usually described as a paradox, devised by Austrian physicist Erwin Schrödinger in 1935. It illustrates what he saw as the problem of the Copenhagen interpretation of quantum mechanics applied to everyday objects. The scenario presents a cat that might be...

, illustrating quantum indeterminacy
Quantum indeterminacy
Quantum indeterminacy is the apparent necessary incompleteness in the description of a physical system, that has become one of the characteristics of the standard description of quantum physics...

 through the manipulation of a perfectly sealed environment and a tiny bit of radioactive substance, and Maxwell's demon
Maxwell's demon
In the philosophy of thermal and statistical physics, Maxwell's demon is a thought experiment created by the Scottish physicist James Clerk Maxwell to "show that the Second Law of Thermodynamics has only a statistical certainty." It demonstrates Maxwell's point by hypothetically describing how to...

, in which a supernatural being is instructed to attempt to violate the second law of thermodynamics
Second law of thermodynamics
The second law of thermodynamics is an expression of the tendency that over time, differences in temperature, pressure, and chemical potential equilibrate in an isolated physical system. From the state of thermodynamic equilibrium, the law deduced the principle of the increase of entropy and...

.

Overview


The ancient Greek deiknymi, or thought experiment, "was the most ancient pattern of mathematical proof", and existed before Euclidean mathematics, where the emphasis was on the conceptual, rather than on the experimental part of a thought-experiment. Thus it is that perhaps the key experiment in the history of modern science, again toppling the lofty but inaccurate view of Aristotle
Aristotle
Aristotle was a Greek philosopher and polymath, a student of Plato and teacher of Alexander the Great. His writings cover many subjects, including physics, metaphysics, poetry, theater, music, logic, rhetoric, linguistics, politics, government, ethics, biology, and zoology...

, is Galileo
Galileo Galilei
Galileo Galilei , was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations and support for Copernicanism...

's demonstration that falling objects must fall at the same rate regardless of their masses. This is widely thought 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 clearly a logical demonstration, using the 'thought experiment' technique. The 'experiment' is described by Galileo in Discorsi e dimostrazioni matematiche
Two New Sciences
The Discourses and Mathematical Demonstrations Relating to Two New Sciences was Galileo's final book and a sort of scientific testament covering much of his work in physics over the preceding thirty years.After his Dialogue Concerning the Two Chief World Systems, the Roman Inquisition had banned...

(1638) (literally, 'Mathematical Discourses and Demonstrations') thus:
Although the extract does not convey the elegance and power of the 'demonstration' terribly well, it is clear that it is a 'thought' experiment, rather than a practical one. Strange then, as Cohen says, that philosophers and scientists alike refuse to acknowledge either Galileo in particular, or the thought experiment technique in general for its pivotal role in both science and philosophy. (The exception proves the rule — the iconoclastic philosopher of science, Paul Feyerabend
Paul Feyerabend
Paul Karl Feyerabend was an Austrian-born philosopher of science best known for his work as a professor of philosophy at the University of California, Berkeley, where he worked for three decades . He lived a peripatetic life, living at various times in England, the United States, New Zealand,...

, has also observed this methodological prejudice.)

Instead, many philosophers prefer to consider 'Thought Experiments' to be merely the use of a hypothetical scenario
Scenario
A scenario is a synoptical collage of an event or series of actions and events. In the Commedia dell'arte it was an outline of entrances, exits, and action describing the plot of a play that was literally pinned to the back of the scenery...

 to help understand the way things actually are.

Variety


There are many different kinds of thought experiments. All thought experiments, however, employ a methodology
Methodology
Methodology is generally a guideline for solving a problem, with specificcomponents such as phases, tasks, methods, techniques and tools . It can be defined also as follows:...

 that is a priori
A priori and a posteriori (philosophy)
The terms a priori and a posteriori are used in philosophy to distinguish two types of knowledge, justifications or arguments...

, rather than empirical
Empirical
The word empirical denotes information gained by means of observation or experimentation. Empirical data are data produced by an experiment or observation....

, in that they do not proceed by observation
Observation
Observation is either an activity of a living being, such as a human, consisting of receiving knowledge of the outside world through the senses, or the recording of data using scientific instruments. The term may also refer to any data collected during this activity...

 or physical experiment
Experiment
An experiment is a methodical procedure carried out with the goal of verifying, falsifying, or establishing the validity of a hypothesis. Experiments vary greatly in their goal and scale, but always rely on repeatable procedure and logical analysis of the results...

.

Thought experiments have been used in a variety of fields, including philosophy
Philosophy
Philosophy is the study of general and fundamental problems, such as those connected with existence, knowledge, values, reason, mind, and language. Philosophy is distinguished from other ways of addressing such problems by its critical, generally systematic approach and its reliance on rational...

, law
Law
Law is a system of rules and guidelines which are enforced through social institutions to govern behavior, wherever possible. It shapes politics, economics and society in numerous ways and serves as a social mediator of relations between people. Contract law regulates everything from buying a bus...

, physics
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...

, and mathematics
Mathematics
Mathematics is the study of quantity, space, structure, and change. Mathematicians seek out patterns and formulate new conjectures. Mathematicians resolve the truth or falsity of conjectures by mathematical proofs, which are arguments sufficient to convince other mathematicians of their validity...

. In philosophy, they have been used at least since classical antiquity
Classical antiquity
Classical antiquity is a broad term for a long period of cultural history centered on the Mediterranean Sea, comprising the interlocking civilizations of ancient Greece and ancient Rome, collectively known as the Greco-Roman world...

, some pre-dating Socrates
Socrates
Socrates was a classical Greek Athenian philosopher. Credited as one of the founders of Western philosophy, he is an enigmatic figure known chiefly through the accounts of later classical writers, especially the writings of his students Plato and Xenophon, and the plays of his contemporary ...

. In law, they were well-known to Roman law
Roman law
Roman law is the legal system of ancient Rome, and the legal developments which occurred before the 7th century AD — when the Roman–Byzantine state adopted Greek as the language of government. The development of Roman law comprises more than a thousand years of jurisprudence — from the Twelve...

yers quoted in the Digest
Pandects
The Digest, also known as the Pandects , is a name given to a compendium or digest of Roman law compiled by order of the emperor Justinian I in the 6th century .The Digest was one part of the Corpus Juris Civilis, the body of civil law issued under Justinian I...

. 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
Galileo Galilei
Galileo Galilei , was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations and support for Copernicanism...

.

Origins and use of the literal term


Johann Witt-Hansen established that Hans Christian Ørsted
Hans Christian Ørsted
Hans Christian Ørsted was a Danish physicist and chemist who discovered that electric currents create magnetic fields, an important aspect of electromagnetism...

 was the first to use the Latin-German mixed term Gedankenexperiment (lit. thought experiment) circa 1812. Ørsted was also the first to use its entirely German equivalent, Gedankenversuch, in 1820.

Much later, Ernst Mach
Ernst Mach
Ernst Mach was an Austrian physicist and philosopher, noted for his contributions to physics such as the Mach number and the study of shock waves...

 used the term Gedankenexperiment in a different way, to denote exclusively the imaginary conduct of a real experiment that would be subsequently performed as a real physical experiment by his students. Physical and mental experimentation could then be constrasted: 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 term thought experiment was coined (as a calque
Calque
In linguistics, a calque or loan translation is a word or phrase borrowed from another language by literal, word-for-word or root-for-root translation.-Calque:...

) from Mach's Gedankenexperiment, and it first appeared in the 1897 English translation of one of Mach’s papers. 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.

Uses


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
Herman Kahn
Herman Kahn was one of the preeminent futurists of the latter third of the twentieth century. In the early 1970s he predicted the rise of Japan as a major world power. He was a founder of the Hudson Institute think tank and originally came to prominence as a military strategist and systems...

’s "scenarios", understand something about something in the future). The understanding comes through reflection upon this imaginary situation. Thought experimentation is a priori
A priori and a posteriori (philosophy)
The terms a priori and a posteriori are used in philosophy to distinguish two types of knowledge, justifications or arguments...

, rather than an empirical
Empirical
The word empirical denotes information gained by means of observation or experimentation. Empirical data are data produced by an experiment or observation....

 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 hypothetical questions that employ subjunctive reasoning (irrealis moods
Irrealis moods
Irrealis moods are the main set of grammatical moods that indicate that a certain situation or action is not known to have happened as the speaker is talking.Every language has a formula for the unreal...

) – "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
Socrates
Socrates was a classical Greek Athenian philosopher. Credited as one of the founders of Western philosophy, he is an enigmatic figure known chiefly through the accounts of later classical writers, especially the writings of his students Plato and Xenophon, and the plays of his contemporary ...

 (see ). 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
Philosophy
Philosophy is the study of general and fundamental problems, such as those connected with existence, knowledge, values, reason, mind, and language. Philosophy is distinguished from other ways of addressing such problems by its critical, generally systematic approach and its reliance on rational...

, physics
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...

, and other fields (such as cognitive psychology
Cognitive psychology
Cognitive psychology is a subdiscipline of psychology exploring internal mental processes.It is the study of how people perceive, remember, think, speak, and solve problems.Cognitive psychology differs from previous psychological approaches in two key ways....

, history
History
History is the discovery, collection, organization, and presentation of information about past events. History can also mean the period of time after writing was invented. Scholars who write about history are called historians...

, political science
Political science
Political Science is a social science discipline concerned with the study of the state, government and politics. Aristotle defined it as the study of the state. It deals extensively with the theory and practice of politics, and the analysis of political systems and political behavior...

, economics
Economics
Economics is the social science that analyzes the production, distribution, and consumption of goods and services. The term economics comes from the Ancient Greek from + , hence "rules of the house"...

, social psychology
Social psychology
Social psychology is the scientific study of how people's thoughts, feelings, and behaviors are influenced by the actual, imagined, or implied presence of others. By this definition, scientific refers to the empirical method of investigation. The terms thoughts, feelings, and behaviors include all...

, law
Law
Law is a system of rules and guidelines which are enforced through social institutions to govern behavior, wherever possible. It shapes politics, economics and society in numerous ways and serves as a social mediator of relations between people. Contract law regulates everything from buying a bus...

, organizational studies
Organizational studies
Organizational studies, sometimes known as organizational science, encompass the systematic study and careful application of knowledge about how people act within organizations...

, marketing
Marketing
Marketing is the process used to determine what products or services may be of interest to customers, and the strategy to use in sales, communications and business development. It generates the strategy that underlies sales techniques, business communication, and business developments...

, and epidemiology
Epidemiology
Epidemiology is the study of health-event, health-characteristic, or health-determinant patterns in a population. It is the cornerstone method of public health research, and helps inform policy decisions and evidence-based medicine by identifying risk factors for disease and targets for preventive...

). 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
    Reductio ad absurdum
    In logic, proof by contradiction is a form of proof that establishes the truth or validity of a proposition by showing that the proposition's being false would imply a contradiction...

    , (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 in order to:
  • challenge the prevailing status quo
    Status quo
    Statu quo, a commonly used form of the original Latin "statu quo" – literally "the state in which" – is a Latin term meaning the current or existing state of affairs. To maintain the status quo is to keep the things the way they presently are...

     (which includes activities such as correcting misinformation
    Misinformation
    Misinformation is false or inaccurate information that is spread unintentionally. It is distinguished from disinformation by motive in that misinformation is simply erroneous, while disinformation, in contrast, is intended to mislead....

     (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
    Forecasting
    Forecasting is the process of making statements about events whose actual outcomes have not yet been observed. A commonplace example might be estimation for some variable of interest at some specified future date. Prediction is a similar, but more general term...

     the (otherwise) indefinite and unknowable future;
  • explain the past;
  • the retrodiction
    Retrodiction
    Retrodiction is the act of making a "prediction" about the past. This is especially useful when one wishes to test a theory whose actual predictions are too long-term to be of immediate use...

    , postdiction
    Postdiction
    According to critics of paranormal beliefs, postdiction is an effect of hindsight bias that explains claimed predictions of significant events, such as plane crashes and natural disasters...

     and hindcast
    Hindcast
    A hindcast is a way of testing a mathematical model. Known or closely estimated inputs for past events are entered into the model to see how well the output matches the known results. Hindcasting is also known as backtesting....

    ing 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
    Functional fixedness
    Functional fixedness is a cognitive bias that limits a person to using an object only in the way it is traditionally used. The concept of functional fixedness originated in Gestalt Psychology, a movement in psychology that emphasizes holistic processing...

    );
  • attribute causation, preventability, blame and responsibility for specific outcomes;
  • assess culpability
    Culpability
    Culpability descends from the Latin concept of fault . The concept of culpability is intimately tied up with notions of agency, freedom and free will...

     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.

In science


Scientists tend to use thought experiments in the form of imaginary, "proxy" experiments which they conduct prior to a real, "physical" experiment (Ernst Mach
Ernst Mach
Ernst Mach was an Austrian physicist and philosopher, noted for his contributions to physics such as the Mach number and the study of shock waves...

 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
Carl Gustav Hempel
Carl Gustav "Peter" Hempel was a philosopher of science and a major figure in 20th-century logical empiricism...

 labeled these sorts of experiment "theoretical experiments-in-imagination"), such as Einstein's thought experiment of chasing a light beam, leading to Special Relativity
Special relativity
Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...

. 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.

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 famous paper on a newly-created subject called later the EPR effect (EPR paradox
EPR paradox
The EPR paradox is a topic in quantum physics and the philosophy of science concerning the measurement and description of microscopic systems by the methods of quantum physics...

). 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
Niels Bohr
Niels Henrik David Bohr was a Danish physicist who made foundational contributions to understanding atomic structure and quantum mechanics, for which he received the Nobel Prize in Physics in 1922. Bohr mentored and collaborated with many of the top physicists of the century at his institute in...

 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 empirical fact (e.g. by the optical real experiments of Alain Aspect
Alain Aspect
Alain Aspect is a French physicist noted for his experimental work on quantum entanglement....

).

Thus thought experiments belong to a theoretical discipline, usually to theoretical physics
Theoretical physics
Theoretical physics is a branch of physics which employs mathematical models and abstractions of physics to rationalize, explain and predict natural phenomena...

, but often to theoretical philosophy
Theoretical philosophy
The division of philosophy into a practical and a theoretical discipline has its origin in Aristotle's moral philosophy and natural philosophy categories. In Denmark, Finland, Poland, and Sweden courses in theoretical and practical philosophy are taught separately, and are separate degrees...

. 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.

Causal reasoning


Generally speaking, there are seven types of thought experiments in which one reasons from causes to effects, or effects to causes :

Prefactual


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

Counterfactual


Counterfactual
Counterfactual conditional
A counterfactual conditional, subjunctive conditional, or remote conditional, abbreviated , is a conditional statement indicating what would be the case if its antecedent were true...

 (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
Isaac Newton
Sir Isaac Newton PRS was an English physicist, mathematician, astronomer, natural philosopher, alchemist, and theologian, who has been "considered by many to be the greatest and most influential scientist who ever lived."...

 and Gottfried Leibniz
Gottfried Leibniz
Gottfried Wilhelm Leibniz was a German philosopher and mathematician. He wrote in different languages, primarily in Latin , French and German ....

  had cooperated with each other, what would mathematics look like today?").

Semifactual


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 traveling at such a speed?).

Semifactual speculations are an important part of clinical medicine.

Prediction


The activity of prediction
Prediction
A prediction or forecast is a statement about the way things will happen in the future, often but not always based on experience or knowledge...

 attempts to project the circumstances of the present into the future.

Hindcasting


The activity of hindcast
Hindcast
A hindcast is a way of testing a mathematical model. Known or closely estimated inputs for past events are entered into the model to see how well the output matches the known results. Hindcasting is also known as backtesting....

ing 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
Reverse engineering
Reverse engineering is the process of discovering the technological principles of a device, object, or system through analysis of its structure, function, and operation...

 and Forensics
Forensics
Forensic science is the application of a broad spectrum of sciences to answer questions of interest to a legal system. This may be in relation to a crime or a civil action...

).

Backcasting


The activity of backcasting
Backcasting
Backcasting starts with defining a desirable future and then works backwards to identify policies and programs that will connect the future to the present. The fundamental question of backcasting asks: "if we want to attain a certain goal, what actions must be taken to get there?"Forecasting is the...

involves 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.

In philosophy


Whereas thought experiments in physics are intended to give us a priori
A priori and a posteriori (philosophy)
The terms a priori and a posteriori are used in philosophy to distinguish two types of knowledge, justifications or arguments...

knowledge of the natural world, philosophy attempts to produce a priori knowledge of our concepts:
[Philosophical and scientific] investigations differ both in their methods (the former is a priori, and the latter a posteriori) and in the metaphysical status of their results (the former yields facts that are metaphysically necessary and the latter yields facts that are metaphysically contingent). Yet the two types of investigations resemble each other in that both, if successful, uncover new facts, and these facts, although expressed in language, are generally not about language (except for investigations in such specialized areas as philosophy of language and empirical linguistics).


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 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
Consequentialism
Consequentialism is the class of normative ethical theories holding that the consequences of one's conduct are the ultimate basis for any judgment about the rightness of that conduct...

). John Searle
John Searle
John Rogers Searle is an American philosopher and currently the Slusser Professor of Philosophy at the University of California, Berkeley.-Biography:...

 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
Functionalism (philosophy of mind)
Functionalism is a theory of the mind in contemporary philosophy, developed largely as an alternative to both the identity theory of mind and behaviourism. Its core idea is that mental states are constituted solely by their functional role — that is, they are causal relations to other mental...

 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, John Rawls
John Rawls
John Bordley Rawls was an American philosopher and a leading figure in moral and political philosophy. He held the James Bryant Conant University Professorship at Harvard University....

 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 (See the veil of ignorance). The use of the state of nature
State of nature
State of nature is a term in political philosophy used in social contract theories to describe the hypothetical condition that preceded governments...

 to imagine the origins of government, as by Thomas Hobbes and John Locke
John Locke
John Locke FRS , widely known as the Father of Liberalism, was an English philosopher and physician regarded as one of the most influential of Enlightenment thinkers. Considered one of the first of the British empiricists, following the tradition of Francis Bacon, he is equally important to social...

, may also be considered a thought experiment. Similarly, 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
Plato
Plato , was a Classical Greek philosopher, mathematician, student of Socrates, writer of philosophical dialogues, and founder of the Academy in Athens, the first institution of higher learning in the Western world. Along with his mentor, Socrates, and his student, Aristotle, Plato helped to lay the...

's allegory of the cave
Allegory of the cave
The Allegory of the Cave—also known as the Analogy of the Cave, Plato's Cave, or the Parable of the Cave—is an allegory used by the Greek philosopher Plato in his work The Republic to illustrate "our nature in its education and want of education"...

. Another historic thought experiment was Avicenna
Avicenna
Abū ʿAlī al-Ḥusayn ibn ʿAbd Allāh ibn Sīnā , commonly known as Ibn Sīnā or by his Latinized name Avicenna, was a Persian polymath, who wrote almost 450 treatises on a wide range of subjects, of which around 240 have survived...

'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
Self-awareness
Self-awareness is the capacity for introspection and the ability to reconcile oneself as an individual separate from the environment and other individuals...

 and self-consciousness
Self-consciousness
Self-consciousness is an acute sense of self-awareness. It is a preoccupation with oneself, as opposed to the philosophical state of self-awareness, which is the awareness that one exists as an individual being; although some writers use both terms interchangeably or synonymously...

, and the substantiality
Substance theory
Substance theory, or substance attribute theory, is an ontological theory about objecthood, positing that a substance is distinct from its properties. A thing-in-itself is a property-bearer that must be distinguished from the properties it bears....

 of the soul.

Possibility


The scenario presented in a thought experiment must be possible in some sense. In many thought experiments, the scenario would be nomologically possible, or possible according to the laws of nature. John Searle's Chinese Room
Chinese room
The Chinese room is a thought experiment by John Searle, which first appeared in his paper "Minds, Brains, and Programs", published in Behavioral and Brain Sciences in 1980...

 is nomologically possible.

Some thought experiments present scenarios that are not nomologically
Subjunctive possibility
Subjunctive possibility is the form of modality most frequently studied in modal logic...

 possible. In his Twin Earth thought experiment
Twin Earth thought experiment
The Twin Earth thought experiment was presented by philosopher Hilary Putnam in his 1973 paper "Meaning and Reference" and subsequent 1975 paper "The Meaning of 'Meaning'", as an early argument for what has subsequently come to be known as semantic externalism...

, Hilary Putnam
Hilary Putnam
Hilary Whitehall Putnam is an American philosopher, mathematician and computer scientist, who has been a central figure in analytic philosophy since the 1960s, especially in philosophy of mind, philosophy of language, philosophy of mathematics, and philosophy of science...

 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 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
David Chalmers
David John Chalmers is an Australian philosopher specializing in the area of philosophy of mind and philosophy of language, whose recent work concerns verbal disputes. He is Professor of Philosophy and Director of the Centre for Consciousness at the Australian National University...

 says that we can imagine that there are zombies
Philosophical zombie
A philosophical zombie or p-zombie in the philosophy of mind and perception is a hypothetical being that is indistinguishable from a normal human being except in that it lacks conscious experience, qualia, or sentience...

, or persons who are physically identical to us in every way but who lack consciousness. This is supposed to show that physicalism
Physicalism
Physicalism is a philosophical position holding that everything which exists is no more extensive than its physical properties; that is, that there are no kinds of things other than physical things...

 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.

Other criticisms


The use of thought experiments in philosophy has received other criticisms, especially in the philosophy of mind
Philosophy of mind
Philosophy of mind is a branch of philosophy that studies the nature of the mind, mental events, mental functions, mental properties, consciousness and their relationship to the physical body, particularly the brain. The mind-body problem, i.e...

. Daniel Dennett
Daniel Dennett
Daniel Clement Dennett is an American philosopher, writer and cognitive scientist whose research centers on the philosophy of mind, philosophy of science and philosophy of biology, particularly as those fields relate to evolutionary biology and cognitive science. He is currently the Co-director of...

 has derisively referred to certain types of thought experiments such as the Chinese Room experiment as "intuition pump
Intuition pump
An intuition pump is a thought experiment structured to elicit intuitive answers about a problem. The term was coined by Daniel Dennett. In Consciousness Explained, he uses the term pejoratively to describe John Searle's Chinese Room thought experiment, characterizing it as designed to elicit...

s", claiming they are simply thinly veiled appeals to intuition which fail when carefully analyzed. Another 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.

Physics


Thought experiments are popular in physics
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...

 and include:
  • Bell's spaceship paradox
    Bell's spaceship paradox
    Bell's spaceship paradox is a thought experiment in special relativity involving accelerated spaceships and strings. The results of this thought experiment are for many people paradoxical. While J. S. Bell's 1976 version of the paradox is the most widely known, it was first designed by E. Dewan and M...

     (special relativity
    Special relativity
    Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...

    )
  • Brownian ratchet
    Brownian ratchet
    In the philosophy of thermal and statistical physics, the Brownian ratchet, or Feynman-Smoluchowski ratchet is a thought experiment about an apparent perpetual motion machine first analysed in 1912 by Polish physicist Marian Smoluchowski and popularised by American Nobel laureate physicist Richard...

     (Richard Feynman
    Richard Feynman
    Richard Phillips Feynman was an American physicist known for his work in the path integral formulation of quantum mechanics, the theory of quantum electrodynamics and the physics of the superfluidity of supercooled liquid helium, as well as in particle physics...

    's "perpetual motion
    Perpetual motion
    Perpetual motion describes hypothetical machines that operate or produce useful work indefinitely and, more generally, hypothetical machines that produce more work or energy than they consume, whether they might operate indefinitely or not....

    " machine that does not violate the second law and does no work at thermal equilibrium)
  • Bucket argument
    Bucket argument
    Isaac Newton's rotating bucket argument was designed to demonstrate that true rotational motion cannot be defined as the relative rotation of the body with respect to the immediately surrounding bodies...

     – argues that space is absolute, not relational
  • Double-slit experiment
    Double-slit experiment
    The double-slit experiment, sometimes called Young's experiment, is a demonstration that matter and energy can display characteristics of both waves and particles...

     (quantum mechanics
    Quantum mechanics
    Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...

    )
  • Elitzur–Vaidman bomb-tester (quantum mechanics
    Quantum mechanics
    Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...

    )
  • Einstein's box
  • EPR paradox
    EPR paradox
    The EPR paradox is a topic in quantum physics and the philosophy of science concerning the measurement and description of microscopic systems by the methods of quantum physics...

     (quantum mechanics
    Quantum mechanics
    Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...

    ) (forms of this have actually been performed)
  • Feynman sprinkler
    Feynman sprinkler
    A Feynman sprinkler, also referred to as a Feynman inverse sprinkler or as a reverse sprinkler, is a sprinkler-like device which is submerged in a tank and made to suck in the surrounding fluid...

     (classical mechanics)
  • Galileo's ship
    Galileo's ship
    Galileo's ship is a physics experiment proposed by Galileo Galilei, the famous 16th and 17th century physicist, astronomer, and philosopher. The experiment was created to disprove popular arguments against the idea of a rotating Earth.-Background:...

     (classical relativity principle) 1632
  • Galileo's Leaning Tower of Pisa experiment
    Galileo's Leaning Tower of Pisa experiment
    right|thumb|Hammer and Feather Drop - [[Apollo 15]] [[astronaut]] [[David Scott]] on the Moon recreating Galileo's famous experiment....

     (rebuttal of Aristotelian Gravity)
  • GHZ experiment
    GHZ experiment
    GHZ experiments are a class of physics experiments that may be used to generate starkly contrasting predictions from local hidden variable theory and quantum mechanical theory, and permit immediate comparison with actual experimental results. A GHZ experiment is similar to a test of Bell's...

     (quantum mechanics
    Quantum mechanics
    Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...

    )
  • Heisenberg's microscope
    Heisenberg's microscope
    Heisenberg's microscope exists only as a thought experiment, one that was proposed by Werner Heisenberg, criticized by his mentor Niels Bohr, and subsequently served as the nucleus of some commonly held ideas, and misunderstandings, about Quantum Mechanics...

     (quantum mechanics
    Quantum mechanics
    Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...

    )
  • Kepler's Dream (change of point of view as support for the Copernican hypothesis)
  • Ladder paradox
    Ladder paradox
    The ladder paradox is a thought experiment in special relativity. It involves a ladder travelling horizontally and undergoing a length contraction, the result of which being that it can fit into a much smaller garage...

     (special relativity
    Special relativity
    Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...

    )
  • Laplace's demon
    Laplace's demon
    In the history of science, Laplace's demon was the first published articulation of causal or scientific determinism by Pierre-Simon Laplace in 1814...

  • Maxwell's demon
    Maxwell's demon
    In the philosophy of thermal and statistical physics, Maxwell's demon is a thought experiment created by the Scottish physicist James Clerk Maxwell to "show that the Second Law of Thermodynamics has only a statistical certainty." It demonstrates Maxwell's point by hypothetically describing how to...

     (thermodynamics
    Thermodynamics
    Thermodynamics is a physical science that studies the effects on material bodies, and on radiation in regions of space, of transfer of heat and of work done on or by the bodies or radiation...

    ) 1871
  • Monkey and the Hunter, The
    The Monkey and the Hunter
    "The Monkey and the Hunter" is a thought experiment often used to illustrate the effect of gravity on projectile motion.The essentials of the problem are stated in many introductory guides to physics, such as Caltech's The Mechanical Universe television series and Gonick and Huffman's Cartoon Guide...

     (gravitation
    Gravitation
    Gravitation, or gravity, is a natural phenomenon by which physical bodies attract with a force proportional to their mass. Gravitation is most familiar as the agent that gives weight to objects with mass and causes them to fall to the ground when dropped...

    )
  • Moving magnet and conductor problem
    Moving magnet and conductor problem
    The moving magnet and conductor problem is a famous thought experiment, originating in the 19th century, concerning the intersection of classical electromagnetism and special relativity. In it, the current in a conductor moving with constant velocity, v, with respect to a magnet is calculated in...

  • Newton's cannonball
    Newton's cannonball
    Newton's cannonball was a thought experiment Isaac Newton used to hypothesize that the force of gravity was universal, and it was the key force for planetary motion...

     (Newton's laws of motion
    Newton's laws of motion
    Newton's laws of motion are three physical laws that form the basis for classical mechanics. They describe the relationship between the forces acting on a body and its motion due to those forces...

    )
  • Popper's experiment
    Popper's experiment
    Popper's experiment is an experiment proposed by the 20th century philosopher of science Karl Popper, an advocate of an objective interpretation of quantum mechanics. He wanted to test the Copenhagen interpretation, a popular subjectivist interpretation of quantum mechanics...

     (quantum mechanics
    Quantum mechanics
    Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...

    )
  • Quantum pseudo telepathy (quantum mechanics
    Quantum mechanics
    Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...

    )
  • Quantum suicide
    Quantum suicide
    In quantum mechanics, quantum suicide is a thought experiment. It was originally published independently by Hans Moravec in 1987 and Bruno Marchal in 1988 and was independently developed further by Max Tegmark in 1998...

     (quantum mechanics
    Quantum mechanics
    Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...

    )
  • Schrödinger's cat
    Schrödinger's cat
    Schrödinger's cat is a thought experiment, usually described as a paradox, devised by Austrian physicist Erwin Schrödinger in 1935. It illustrates what he saw as the problem of the Copenhagen interpretation of quantum mechanics applied to everyday objects. The scenario presents a cat that might be...

     (quantum mechanics
    Quantum mechanics
    Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...

    )
  • Sticky bead argument
    Sticky bead argument
    In general relativity, the sticky bead argument is a simple thought experiment designed to show that gravitational radiation is indeed predicted by general relativity, and can have physical effects...

     (general relativity
    General relativity
    General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...

    )
  • Renninger negative-result experiment
    Renninger negative-result experiment
    In quantum mechanics, the Renninger negative-result experiment is a thought experiment that illustrates some of the difficulties of understanding the nature of wave function collapse and measurement in quantum mechanics...

     (quantum mechanics
    Quantum mechanics
    Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...

    )
  • Twin paradox
    Twin paradox
    In physics, the twin paradox is a thought experiment in special relativity, in which a twin makes a journey into space in a high-speed rocket and returns home to find he has aged less than his identical twin who stayed on Earth...

     (special relativity
    Special relativity
    Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...

    )
  • Wheeler's delayed choice experiment
    Wheeler's delayed choice experiment
    Wheeler's delayed choice experiment is a thought experiment proposed by John Archibald Wheeler in 1978, and later confirmed. Wheeler proposed a variation of the famous double-slit experiment of quantum physics, one in which the method of detection can be changed after the photon passes the double...

     (quantum mechanics
    Quantum mechanics
    Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...

    )
  • Wigner's friend
    Wigner's friend
    Wigner's friend is a thought experiment proposed by the physicist Eugene Wigner; it is an extension of the Schrödinger's cat experiment designed as a point of departure for discussing the Quantum mind/body problem.- The thought experiment :...

     (quantum mechanics
    Quantum mechanics
    Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...

    )

Philosophy


The field of philosophy
Philosophy
Philosophy is the study of general and fundamental problems, such as those connected with existence, knowledge, values, reason, mind, and language. Philosophy is distinguished from other ways of addressing such problems by its critical, generally systematic approach and its reliance on rational...

 makes extensive use of thought experiments:
  • Artificial brain
    Artificial brain
    Artificial brain is a term commonly used in the media to describe research that aims to develop software and hardware with cognitive abilities similar to the animal or human brain...

  • Avicenna
    Avicenna
    Abū ʿAlī al-Ḥusayn ibn ʿAbd Allāh ibn Sīnā , commonly known as Ibn Sīnā or by his Latinized name Avicenna, was a Persian polymath, who wrote almost 450 treatises on a wide range of subjects, of which around 240 have survived...

    's Floating Man
  • Bellum omnium contra omnes
    Bellum omnium contra omnes
    Bellum omnium contra omnes, a Latin phrase meaning "the war of all against all," is the description that Thomas Hobbes gives to human existence in the state of nature thought experiment that he conducts in De Cive and Leviathan ....

  • Big Book
    Big Book (thought experiment)
    The "Big Book" is a thought experiment developed by Ludwig Wittgenstein about the nature of ethics and the verifiability of ethical knowledge.- Summary of The Experiment :...

     (ethics
    Ethics
    Ethics, also known as moral philosophy, is a branch of philosophy that addresses questions about morality—that is, concepts such as good and evil, right and wrong, virtue and vice, justice and crime, etc.Major branches of ethics include:...

    )
  • Brain-in-a-vat (epistemology, philosophy of mind
    Philosophy of mind
    Philosophy of mind is a branch of philosophy that studies the nature of the mind, mental events, mental functions, mental properties, consciousness and their relationship to the physical body, particularly the brain. The mind-body problem, i.e...

    )
  • Brainstorm machine
    Brainstorm machine
    In the philosophy of mind, the Brainstorm machine is a thought experiment described by Daniel Dennett, to show that it is not possible to intersubjectively compare any two individuals' personal experiences, or qualia, even with perfect technology. It is based on a device described in the film...

  • Buridan's ass
    Buridan's ass
    Buridan's ass is an illustration of a paradox in philosophy in the conception of free will.It refers to a hypothetical situation wherein an ass is placed precisely midway between a stack of hay and a pail of water...

  • Changing places
    Changing places
    The changing places thought experiment was conceived of by Max Velmans, Professor of Psychology at Goldsmiths College, University of London, and was discussed in his 2000 work, Understanding Consciousness...

      (reflexive monism
    Reflexive monism
    Monism is the view that the universe, at the deepest level of analysis, is one thing or composed of one fundamental kind of stuff. This is usually contrasted with Substance Dualism, the view found for example in the writings of Plato and Descartes that, fundamentally, the universe is composed of...

    , philosophy of mind
    Philosophy of mind
    Philosophy of mind is a branch of philosophy that studies the nature of the mind, mental events, mental functions, mental properties, consciousness and their relationship to the physical body, particularly the brain. The mind-body problem, i.e...

    )
  • China brain
    China brain
    In the philosophy of mind, the China brain thought experiment considers what would happen if each member of the Chinese nation were asked to simulate the action of one neuron in the brain, using telephones or walkie-talkies to simulate the axons and dendrites that connect neurons...

     (physicalism
    Physicalism
    Physicalism is a philosophical position holding that everything which exists is no more extensive than its physical properties; that is, that there are no kinds of things other than physical things...

    , philosophy of mind
    Philosophy of mind
    Philosophy of mind is a branch of philosophy that studies the nature of the mind, mental events, mental functions, mental properties, consciousness and their relationship to the physical body, particularly the brain. The mind-body problem, i.e...

    )
  • Chinese room
    Chinese room
    The Chinese room is a thought experiment by John Searle, which first appeared in his paper "Minds, Brains, and Programs", published in Behavioral and Brain Sciences in 1980...

     (philosophy of mind
    Philosophy of mind
    Philosophy of mind is a branch of philosophy that studies the nature of the mind, mental events, mental functions, mental properties, consciousness and their relationship to the physical body, particularly the brain. The mind-body problem, i.e...

    , artificial intelligence
    Artificial intelligence
    Artificial intelligence is the intelligence of machines and the branch of computer science that aims to create it. AI textbooks define the field as "the study and design of intelligent agents" where an intelligent agent is a system that perceives its environment and takes actions that maximize its...

    , cognitive science
    Cognitive science
    Cognitive science is the interdisciplinary scientific study of mind and its processes. It examines what cognition is, what it does and how it works. It includes research on how information is processed , represented, and transformed in behaviour, nervous system or machine...

    )
  • Coherence (philosophical gambling strategy)
    Coherence (philosophical gambling strategy)
    In a thought experiment proposed by the Italian probabilist Bruno de Finetti in order to justify Bayesian probability, an array of wagers is coherent precisely if it does not expose the wagerer to certain loss regardless of the outcomes of events on which he is wagering, even if his opponent makes...

  • Condillac
    Étienne Bonnot de Condillac
    Étienne Bonnot de Condillac was a French philosopher and epistemologist who studied in such areas as psychology and the philosophy of the mind.-Biography:...

    's Statue (epistemology)
  • Gettier problem
    Gettier problem
    A Gettier problem is a problem in modern epistemology issuing from counter-examples to the definition of knowledge as justified true belief . The problem owes its name to a three-page paper published in 1963, by Edmund Gettier, called "Is Justified True Belief Knowledge?", in which Gettier argues...

     (epistemology)
  • Ḥayy ibn Yaqẓān
    Hayy ibn Yaqdhan
    Ḥayy ibn Yaqẓān is an Arabic philosophical novel and allegorical tale written by Ibn Tufail in the early 12th century.- Translations :* from Wikisource* English translations of Hayy bin Yaqzan...

     (epistemology)
  • Hilary Putnam
    Hilary Putnam
    Hilary Whitehall Putnam is an American philosopher, mathematician and computer scientist, who has been a central figure in analytic philosophy since the 1960s, especially in philosophy of mind, philosophy of language, philosophy of mathematics, and philosophy of science...

    's Twin Earth thought experiment
    Twin Earth thought experiment
    The Twin Earth thought experiment was presented by philosopher Hilary Putnam in his 1973 paper "Meaning and Reference" and subsequent 1975 paper "The Meaning of 'Meaning'", as an early argument for what has subsequently come to be known as semantic externalism...

     in the philosophy of language
    Philosophy of language
    Philosophy of language is the reasoned inquiry into the nature, origins, and usage of language. As a topic, the philosophy of language for analytic philosophers is concerned with four central problems: the nature of meaning, language use, language cognition, and the relationship between language...

     and philosophy of mind
    Philosophy of mind
    Philosophy of mind is a branch of philosophy that studies the nature of the mind, mental events, mental functions, mental properties, consciousness and their relationship to the physical body, particularly the brain. The mind-body problem, i.e...

  • How many men? (taxation as theft
    Taxation as theft
    The identification of taxation as theft is viewpoint held by anarchists and some non-anarchist libertarians. It states that government is transgressing property rights by enforcing compulsory tax collection.-See also:*Anarchism*Libertarianism*Objectivism...

    )
  • Inverted spectrum
    Inverted spectrum
    Inverted spectrum is the apparent possibility of two people sharing their color vocabulary and discriminations, although the colours one sees — their qualia — are systematically different from the colours the other person sees....

  • Kavka's toxin puzzle
    Kavka's toxin puzzle
    Kavka's toxin puzzle is a thought experiment about the possibility of forming an intention to perform an act which, following from reason, is an action one would not actually perform. It was presented by moral and political philosopher Gregory S...

  • Mary's room
    Mary's room
    Mary's room is a philosophical thought experiment proposed by Frank Jackson in his article "Epiphenomenal Qualia" and extended in "What Mary Didn't Know"...

     (philosophy of mind
    Philosophy of mind
    Philosophy of mind is a branch of philosophy that studies the nature of the mind, mental events, mental functions, mental properties, consciousness and their relationship to the physical body, particularly the brain. The mind-body problem, i.e...

    )
  • Molyneux's Problem
    Molyneux's Problem
    Molyneux's problem is a thought experiment in philosophy concerning immediate recovery from blindness.It was first formulated by William Molyneux, and notably referenced in John Locke's An Essay Concerning Human Understanding....

     (admittedly, this oscillated between empirical and a-priori assessment)
  • Newcomb's paradox
    Newcomb's paradox
    Newcomb's paradox, also referred to as Newcomb's problem, is a thought experiment involving a game between two players, one of whom purports to be able to predict the future. Whether the problem is actually a paradox is disputed....

  • Original position
    Original position
    The original position is a hypothetical situation developed by American philosopher John Rawls as a thought experiment to replace the imagery of a savage state of nature of prior political philosophers like Thomas Hobbes. In it, the parties select principles that will determine the basic structure...

     (politics
    Politics
    Politics is a process by which groups of people make collective decisions. The term is generally applied to the art or science of running governmental or state affairs, including behavior within civil governments, but also applies to institutions, fields, and special interest groups such as the...

    )
  • Philosophical zombie
    Philosophical zombie
    A philosophical zombie or p-zombie in the philosophy of mind and perception is a hypothetical being that is indistinguishable from a normal human being except in that it lacks conscious experience, qualia, or sentience...

     (philosophy of mind
    Philosophy of mind
    Philosophy of mind is a branch of philosophy that studies the nature of the mind, mental events, mental functions, mental properties, consciousness and their relationship to the physical body, particularly the brain. The mind-body problem, i.e...

    , artificial intelligence
    Artificial intelligence
    Artificial intelligence is the intelligence of machines and the branch of computer science that aims to create it. AI textbooks define the field as "the study and design of intelligent agents" where an intelligent agent is a system that perceives its environment and takes actions that maximize its...

    , cognitive science
    Cognitive science
    Cognitive science is the interdisciplinary scientific study of mind and its processes. It examines what cognition is, what it does and how it works. It includes research on how information is processed , represented, and transformed in behaviour, nervous system or machine...

    )
  • Plank of Carneades
    Plank of Carneades
    In ethics, the plank of Carneades is a thought experiment first proposed by Carneades of Cyrene; it explores the concept of self-defense in relation to murder....

  • Prisoner's Dilemma
    Prisoner's dilemma
    The prisoner’s dilemma is a canonical example of a game, analyzed in game theory that shows why two individuals might not cooperate, even if it appears that it is in their best interest to do so. It was originally framed by Merrill Flood and Melvin Dresher working at RAND in 1950. Albert W...

  • Ship of Theseus, The
    Ship of Theseus
    The Ship of Theseus, also known as Theseus' paradox, or various variants, notably grandfather's axe and Trigger's Broom is a paradox that raises the question of whether an object which has had all its component parts replaced remains fundamentally the same object.The paradox is most notably...

     (concept of identity
    Identity (philosophy)
    In philosophy, identity, from , is the relation each thing bears just to itself. According to Leibniz's law two things sharing every attribute are not only similar, but are the same thing. The concept of sameness has given rise to the general concept of identity, as in personal identity and...

    )
  • Simulated reality
    Simulated reality
    Simulated reality is the proposition that reality could be simulated—perhaps by computer simulation—to a degree indistinguishable from "true" reality. It could contain conscious minds which may or may not be fully aware that they are living inside a simulation....

     (philosophy, computer science
    Computer science
    Computer science or computing science is the study of the theoretical foundations of information and computation and of practical techniques for their implementation and application in computer systems...

    , cognitive science
    Cognitive science
    Cognitive science is the interdisciplinary scientific study of mind and its processes. It examines what cognition is, what it does and how it works. It includes research on how information is processed , represented, and transformed in behaviour, nervous system or machine...

    )
  • Social contract
    Social contract
    The social contract is an intellectual device intended to explain the appropriate relationship between individuals and their governments. Social contract arguments assert that individuals unite into political societies by a process of mutual consent, agreeing to abide by common rules and accept...

     theories
  • Survival lottery, The
    The survival lottery
    The Survival Lottery is a thought experiment, proposed by the philosopher John Harris. The basis of the idea is to ask people to imagine if organ donation were expected to save more individuals than it would kill...

      (ethics
    Ethics
    Ethics, also known as moral philosophy, is a branch of philosophy that addresses questions about morality—that is, concepts such as good and evil, right and wrong, virtue and vice, justice and crime, etc.Major branches of ethics include:...

    )
  • Swamp man (personal identity, philosophy of mind
    Philosophy of mind
    Philosophy of mind is a branch of philosophy that studies the nature of the mind, mental events, mental functions, mental properties, consciousness and their relationship to the physical body, particularly the brain. The mind-body problem, i.e...

    )
  • Shoemaker's
    Sydney Shoemaker
    Sydney Shoemaker is an American philosopher. Until his retirement, he was a Susan Linn Sage Professor of Philosophy at Cornell University. He holds a PhD from Cornell and BA from Reed. In 1971, he delivered the John Locke Lectures at Oxford University...

     "Time Without Change" (metaphysics)
  • Ticking time bomb scenario
    Ticking time bomb scenario
    The ticking time bomb scenario is a thought experiment that has been used in the ethics debate over whether torture can ever be justified.Simply stated, the consequentialist argument is that nations, even those such as the United States that legally disallow torture, can justify its use if they...

     (ethics
    Ethics
    Ethics, also known as moral philosophy, is a branch of philosophy that addresses questions about morality—that is, concepts such as good and evil, right and wrong, virtue and vice, justice and crime, etc.Major branches of ethics include:...

    )
  • Teleportation
    Teleportation
    Teleportation is the fictional or imagined process by which matter is instantaneously transferred from one place to another.Teleportation may also refer to:*Quantum teleportation, a method of transmitting quantum data...

     (metaphysics)
  • Trolley problem
    Trolley problem
    The trolley problem is a thought experiment in ethics, first introduced by Philippa Foot, but also extensively analysed by Judith Jarvis Thomson, Peter Unger, and Frances Kamm...

     (ethics
    Ethics
    Ethics, also known as moral philosophy, is a branch of philosophy that addresses questions about morality—that is, concepts such as good and evil, right and wrong, virtue and vice, justice and crime, etc.Major branches of ethics include:...

    )
  • The Violinist
    Violinist (Thought Experiment)
    The Violinist is a famous thought experiment first posed by Judith Jarvis Thomson in 1971.-The "famous violinist" thought experiment:The Stanford Encyclopedia of Philosophy describes her thought experiment as follows:...

     (ethics
    Ethics
    Ethics, also known as moral philosophy, is a branch of philosophy that addresses questions about morality—that is, concepts such as good and evil, right and wrong, virtue and vice, justice and crime, etc.Major branches of ethics include:...

    )
  • Utility monster
    Utility monster
    The utility monster is a thought experiment in the study of ethics. It was created by philosopher Robert Nozick in 1974 as a criticism of utilitarianism....

     (ethics
    Ethics
    Ethics, also known as moral philosophy, is a branch of philosophy that addresses questions about morality—that is, concepts such as good and evil, right and wrong, virtue and vice, justice and crime, etc.Major branches of ethics include:...

    )
  • Zeno's paradoxes
    Zeno's paradoxes
    Zeno's paradoxes are a set of problems generally thought to have been devised by Greek philosopher Zeno of Elea to support Parmenides's doctrine that "all is one" and that, contrary to the evidence of our senses, the belief in plurality and change is mistaken, and in particular that motion is...

     (classical Greek problems of the infinite)

Mathematics

  • Balls and vase problem (infinity and cardinality)
  • Gabriel's Horn
    Gabriel's Horn
    Gabriel's Horn is a geometric figure which has infinite surface area but encloses a finite volume. The name refers to the tradition identifying the Archangel Gabriel as the angel who blows the horn to announce Judgment Day, associating the divine, or infinite, with the finite...

     (infinity)
  • Infinite monkey theorem
    Infinite monkey theorem
    The infinite monkey theorem states that a monkey hitting keys at random on a typewriter keyboard for an infinite amount of time will almost surely type a given text, such as the complete works of William Shakespeare....

     (probability)
  • Lottery paradox
    Lottery paradox
    Henry E. Kyburg, Jr.'s lottery paradox arises from considering a fair 1000 ticket lottery that has exactly one winning ticket. If this much is known about the execution of the lottery it is therefore rational to accept that some ticket will win. Suppose that an event is very likely only if the...

     (probability)
  • Sleeping beauty paradox
    Sleeping Beauty problem
    The Sleeping Beauty problem is a puzzle in probability theory and formal epistemology in which an ideally rational epistemic agent is to be wakened once or twice according to the toss of a coin, and asked her degree of belief for the coin having come up heads....

     (probability)

Computer science

  • Halting problem
    Halting problem
    In computability theory, the halting problem can be stated as follows: Given a description of a computer program, decide whether the program finishes running or continues to run forever...

     (limits of computability)
  • Turing machine
    Turing machine
    A Turing machine is a theoretical device that manipulates symbols on a strip of tape according to a table of rules. Despite its simplicity, a Turing machine can be adapted to simulate the logic of any computer algorithm, and is particularly useful in explaining the functions of a CPU inside a...

     (limits of computability)
  • Two Generals' Problem
    Two Generals' Problem
    In computing, the Two Generals' Problem is a thought experiment meant to illustrate the pitfalls and design challenges of attempting to coordinate an action by communicating over an unreliable link...

  • Dining Philosophers
    Dining philosophers problem
    In computer science, the dining philosophers problem is an example problem often used in concurrent algorithm design to illustrate synchronization issues and techniques for resolving them....

     (computer science
    Computer science
    Computer science or computing science is the study of the theoretical foundations of information and computation and of practical techniques for their implementation and application in computer systems...

    )

Miscellaneous

  • Buttered cat paradox
    Buttered cat paradox
    The buttered cat paradox is a paradox based on the tongue-in-cheek combination of two adages:* Cats always land on their feet.* Buttered toast always lands buttered side down....

  • Braitenberg vehicles (robotics, neural control and sensing systems) (some have actually been built)
  • Doomsday argument
    Doomsday argument
    The Doomsday argument is a probabilistic argument that claims to predict the number of future members of the human species given only an estimate of the total number of humans born so far...

     (anthropic principle
    Anthropic principle
    In astrophysics and cosmology, the anthropic principle is the philosophical argument that observations of the physical Universe must be compatible with the conscious life that observes it. Some proponents of the argument reason that it explains why the Universe has the age and the fundamental...

    )
  • Dyson sphere
    Dyson sphere
    A Dyson sphere is a hypothetical megastructure originally described by Freeman Dyson. Such a "sphere" would be a system of orbiting solar power satellites meant to completely encompass a star and capture most or all of its energy output...

  • The Lady, or the Tiger? (human nature)
  • The Planiverse
    The Planiverse
    -Development:In 1977, Dewdney was inspired by an allegory of a two-dimensional universe, and decided to expand upon the physics and chemistry of such a universe.He published a short monograph in 1979 called Two-Dimensional Science and Technology.In July 1980, this was reviewed by Martin Gardner in...


Significant articles

  • Dennett, D.C., "Intuition Pumps", pp. 180–197 in Brockman, J., The Third Culture: Beyond the Scientific Revolution, Simon & Schuster, (New York), 1995.
  • 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.
  • Kuhn, T.
    Thomas Kuhn
    Thomas Samuel Kuhn was an American historian and philosopher of science whose controversial 1962 book The Structure of Scientific Revolutions was deeply influential in both academic and popular circles, introducing the term "paradigm shift," which has since become an English-language staple.Kuhn...

     "A Function for Thought Experiments", in The Essential Tension (Chicago: University of Chicago Press, 1979), pp. 240–
  • Mach, E.
    Ernst Mach
    Ernst Mach was an Austrian physicist and philosopher, noted for his contributions to physics such as the Mach number and the study of shock waves...

    , "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.
    Karl Popper
    Sir Karl Raimund Popper, CH FRS FBA was an Austro-British philosopher and a professor at the London School of Economics...

    , "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. Ørsted, Immanuel Kant and the Thought Experiment", Danish Yearbook of Philosophy, Vol.13, (1976), pp. 48–65.
  • Jacques, V., Wu, E., Grosshans, F., Treussart, F., Grangier, P. Aspect, A., & Roch, J. (2007). Experimental Realization of Wheeler's Delayed-Choice Gedanken Experiment, Science, 315, p. 966–968. http://www.sciencemag.org/cgi/content/abstract/315/5814/966

Books

  • Adams, Scott, God's Debris: A Thought Experiment, Andrews McMeel Publishing, (USA), 2001
  • 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.
  • Buzzoni, M., Thought Experiment in the Natural Sciences, Koenigshausen+Neumann, Wuerzburg 2008
  • Cohen, Martin, "Wittgenstein's Beetle and Other Classic Thought Experiments", Blackwell (Oxford) 2005
  • Cohnitz, D., Gedankenexperimente in der Philosophie, Mentis Publ., (Paderborn, Germany), 2006.
  • 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.
  • Kuhne, U., Die Methode des Gedankenexperiments, Suhrkamp Publ., (Frankfurt/M, Germany), 2005.
  • Leatherdale, W.H., The Role of Analogy, Model and Metaphor in Science, North-Holland Publishing Company, (Amsterdam), 1974.. Translated to English by Karen Jelved, Andrew D. Jackson, and Ole Knudsen, (translators 1997).
  • 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.

External links


  • Thought experimentsStanford Encyclopedia of Philosophy
    Stanford Encyclopedia of Philosophy
    The Stanford Encyclopedia of Philosophy is a freely-accessible online encyclopedia of philosophy maintained by Stanford University. Each entry is written and maintained by an expert in the field, including professors from over 65 academic institutions worldwide...

  • Philosophy Bites podcast: Nigel Warburton
    Nigel Warburton
    Nigel Warburton is a philosopher, currently Senior Lecturer at the Open University. He is best known as a populariser of philosophy, being author of a number of books of this genre, but he has also written academic works in esthetics and applied ethics.-Education:Warburton received a BA from the...

     interviews Julian Baggini
    Julian Baggini
    Julian Baggini is the author of several books about philosophy written for a general audience. He is the author of The Pig that Wants to be Eaten and 99 other thought experiments and is co-founder and editor-in-chief of The Philosophers' Magazine...

    on Thought Experiments
  • Stevinus, Galileo, and Thought Experiments Short essay by S. Abbas Raza of 3 Quarks Daily
  • Thought experiment generator, an entertaining visual aid to running your own thought experiment
  • Articles on Thought Experiments in the PhilSci Archive, an electronic archive for preprints in the philosophy of science.