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A thought experiment is a hypothetical situation in which a
hypothesis A hypothesis (plural hypotheses) is a proposed explanation for a phenomenon. For a hypothesis to be a scientific hypothesis, the scientific method requires that one can test it. Scientists generally base scientific hypotheses on previous obse ...
,
theory A theory is a rational type of abstract thinking about a phenomenon, or the results of such thinking. The process of contemplative and rational thinking is often associated with such processes as observational study or research. Theories may be s ...
, or
principle A principle is a proposition or value that is a guide for behavior or evaluation. In law, it is a Legal rule, rule that has to be or usually is to be followed. It can be desirably followed, or it can be an inevitable consequence of something, suc ...
is laid out for the purpose of
thinking In their most common sense, the terms thought and thinking refer to conscious cognitive processes that can happen independently of sensory stimulation. Their most paradigmatic forms are judging, reasoning, concept formation, problem solving, an ...
through its consequences.


History

The ancient Greek ''deiknymi'' (), or thought experiment, "was the most ancient pattern of
mathematical proof A mathematical proof is an inferential argument for a mathematical statement, showing that the stated assumptions logically guarantee the conclusion. The argument may use other previously established statements, such as theorems; but every proo ...
", and existed before Euclidean mathematics, where the emphasis was on the conceptual, rather than on the experimental part of a thought-experiment. Johann Witt-Hansen established that
Hans Christian Ørsted Hans Christian Ørsted ( , ; often rendered Oersted in English; 14 August 17779 March 1851) was a Danish physicist and chemist who discovered that electric currents create magnetic fields, which was the first connection found between electricity ...
was the first to use the German term ' (lit. thought experiment) circa 1812. Ørsted was also the first to use the equivalent term ' in 1820. By 1883
Ernst Mach Ernst Waldfried Josef Wenzel Mach ( , ; 18 February 1838 – 19 February 1916) was a Moravian-born Austrian physicist and philosopher, who contributed to the physics of shock waves. The ratio of one's speed to that of sound is named the Mach ...
used the term ' in a different way, to denote exclusively the conduct of a experiment that would be subsequently performed as a by his students. Physical and mental experimentation could then be contrasted: Mach asked his students to provide him with explanations whenever the results from their subsequent, real, physical experiment differed from those of their prior, imaginary experiment. The English term ''thought experiment'' was coined (as a
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. When used as a verb, "to calque" means to borrow a word or phrase from another language wh ...
) from Mach's ', 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). The
irrealis moods In linguistics, irrealis moods (abbreviated ) are the main set of grammatical moods that indicate that a certain situation or action is not known to have happened at the moment the speaker is talking. This contrasts with the realis moods. Every ...
are ways to categorize it or to speak about it. This helps explain the extremely wide and diverse range of the application of the term "thought experiment" once it had been introduced into English. Perhaps the key experiment in the history of modern science is
Galileo Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642) was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Commonly referred to as Galileo, his name was pronounced (, ). He was ...
'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 heavyweights off it, whereas in fact, it was a logical demonstration, using the 'thought experiment' technique. The 'experiment' is described by Galileo in '' Discorsi e dimostrazioni matematiche'' (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 Karl Feyerabend (; January 13, 1924 – February 11, 1994) 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 (1958 ...
, has also observed this methodological prejudice.) Instead, many philosophers prefer to consider 'Thought Experiments' to be merely the use of a hypothetical
scenario In the performing arts, 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, and was literally pi ...
to help understand the way things are.


Uses

The common goal of a thought experiment is to explore the potential consequences of the principle in question: Given the structure of the experiment, it may not be possible to perform it, and even if it could be performed, there need not be an intention to perform it. Examples of thought experiments include
Schrödinger's cat In quantum mechanics, Schrödinger's cat is a thought experiment that illustrates a paradox of quantum superposition. In the thought experiment, a hypothetical cat may be considered simultaneously both alive and dead, while it is unobserved in ...
, illustrating
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. Prior to quantum physics, it was thought that : ...
through the manipulation of a perfectly sealed environment and a tiny bit of
radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consid ...
substance, and
Maxwell's demon Maxwell's demon is a thought experiment that would hypothetically violate the second law of thermodynamics. It was proposed by the physicist James Clerk Maxwell in 1867. In his first letter Maxwell called the demon a "finite being", while the ' ...
, which attempts to demonstrate the ability of a hypothetical finite being to violate the
2nd law of thermodynamics The second law of thermodynamics is a physical law based on universal experience concerning heat and Energy transformation, energy interconversions. One simple statement of the law is that heat always moves from hotter objects to colder objects ( ...
. It is a common element of
science-fiction Science fiction (sometimes shortened to Sci-Fi or SF) is a genre of speculative fiction which typically deals with imaginative and futuristic concepts such as advanced science and technology, space exploration, time travel, parallel univers ...
stories. Thought experiments, which are well-structured, well-defined hypothetical questions that employ
subjunctive The subjunctive (also known as conjunctive in some languages) is a grammatical mood, a feature of the utterance that indicates the speaker's attitude towards it. Subjunctive forms of verbs are typically used to express various states of unreality ...
reasoning (
irrealis moods In linguistics, irrealis moods (abbreviated ) are the main set of grammatical moods that indicate that a certain situation or action is not known to have happened at the moment the speaker is talking. This contrasts with the realis moods. Every ...
) – "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 (; ; –399 BC) was a Greek philosopher from Athens who is credited as the founder of Western philosophy and among the first moral philosophers of the ethical tradition of thought. An enigmatic figure, Socrates authored no te ...
. In physics and other sciences many thought experiments date from the 19th and especially the 20th Century, but examples can be found at least as early as
Galileo Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642) was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Commonly referred to as Galileo, his name was pronounced (, ). He was ...
. In thought experiments, we gain new information by rearranging or reorganizing already known empirical data in a new way and drawing new (a priori) inferences from them or by looking at these data from a different and unusual perspective. In Galileo's thought experiment, for example, the rearrangement of empirical experience consists of the original idea of combining bodies of different weights. Thought experiments have been used in philosophy (especially ethics),
physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which r ...
, and other fields (such as
cognitive psychology Cognitive psychology is the scientific study of mental processes such as attention, language use, memory, perception, problem solving, creativity, and reasoning. Cognitive psychology originated in the 1960s in a break from behaviorism, which ...
, history,
political science Political science is the scientific study of politics. It is a social science dealing with systems of governance and power, and the analysis of political activities, political thought, political behavior, and associated constitutions and la ...
, economics,
social psychology Social psychology is the scientific study of how thoughts, feelings, and behaviors are influenced by the real or imagined presence of other people or by social norms. Social psychologists typically explain human behavior as a result of the r ...
, law,
organizational studies Organization studies (also called organization science or organizational studies) is the academic field interested in a ''collective activity, and how it relates to organization, organizing, and management''. It is "the examination of how individua ...
, marketing, and
epidemiology Epidemiology is the study and analysis of the distribution (who, when, and where), patterns and determinants of health and disease conditions in a defined population. It is a cornerstone of public health, and shapes policy decisions and evidenc ...
). 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 In logic, (Latin for "reduction to absurdity"), also known as (Latin for "argument to absurdity") or ''apagogical arguments'', is the form of argument that attempts to establish a claim by showing that the opposite scenario would lead to absu ...
, (as in Galileo's original argument, a
proof by contradiction In logic and mathematics, proof by contradiction is a form of proof that establishes the truth or the validity of a proposition, by showing that assuming the proposition to be false leads to a contradiction. Proof by contradiction is also known as ...
), * confirm a prevailing theory, * establish a new theory, or * simultaneously refute a prevailing theory and establish a new theory through a process of mutual exclusion


Practical applications

Thought experiments can produce some very important and different outlooks on previously unknown or unaccepted theories. However, they may make those theories themselves irrelevant, and could possibly create new problems that are just as difficult, or possibly more difficult to resolve. In terms of their practical application, thought experiments are generally created to: * challenge the prevailing status quo (which includes activities such as correcting
misinformation Misinformation is incorrect or misleading information. It differs from disinformation, which is ''deliberately'' deceptive. Rumors are information not attributed to any particular source, and so are unreliable and often unverified, but can turn ou ...
(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 In mathematics, extrapolation is a type of estimation, beyond the original observation range, of the value of a variable on the basis of its relationship with another variable. It is similar to interpolation, which produces estimates between kn ...
beyond (or
interpolate In the mathematical field of numerical analysis, interpolation is a type of estimation, a method of constructing (finding) new data points based on the range of a discrete set of known data points. In engineering and science, one often has a n ...
within) the boundaries of already established fact; *
predict A prediction (Latin ''præ-'', "before," and ''dicere'', "to say"), or forecast, is a statement about a future event or data. They are often, but not always, based upon experience or knowledge. There is no universal agreement about the exact ...
and forecast the (otherwise) indefinite and unknowable future; * explain the past; * the
retrodiction Retrodiction is the act of making a prediction about the past. It is also known as postdiction (but this should not be confused with the use of the term in criticisms of parapsychological research). Activity The activity of retrodiction (or po ...
,
postdiction Postdiction involves explanation after the fact. In skepticism, it is considered an effect of hindsight bias that explains claimed predictions of significant events such as plane crashes and natural disasters. In religious contexts, theologian ...
and
hindcast Backtesting is a term used in modeling to refer to testing a predictive model on historical data. Backtesting is a type of retrodiction, and a special type of cross-validation applied to previous time period(s). Financial analysis In a tradin ...
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.:
functional fixedness Functional fixedness is a cognitive bias that limits a person to use 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 proces ...
); * attribute causation, preventability, blame, and responsibility for specific outcomes; * assess
culpability In criminal law, culpability, or being culpable, is a measure of the degree to which an agent, such as a person, can be held morally or legally responsible for action and inaction. It has been noted that the word, culpability, "ordinarily has ...
and
compensatory damages At common law, damages are a remedy in the form of a monetary award to be paid to a claimant as compensation for loss or injury. To warrant the award, the claimant must show that a breach of duty has caused foreseeable loss. To be recognised at ...
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


Types

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'' – the term prefactual was coined by Lawrence J. Sanna in 1998 — speculate on possible future outcomes, given the present, and ask "What will be the outcome if event E occurs?"


Counterfactual

''Counterfactual (contrary to established fact) thought experiments'' – the term ''
counterfactual Counterfactual conditionals (also ''subjunctive'' or ''X-marked'') are conditional sentences which discuss what would have been true under different circumstances, e.g. "If Peter believed in ghosts, he would be afraid to be here." Counterfactual ...
'' was coined by
Nelson Goodman Henry Nelson Goodman (7 August 1906 – 25 November 1998) was an American philosopher, known for his work on counterfactuals, mereology, the problem of induction, irrealism, and aesthetics. Life and career Goodman was born in Somerville, Mas ...
in 1947,Goodman, N., "The Problem of Counterfactual Conditionals", ''The Journal of Philosophy'', Vol.44, No.5, (27 February 1947), pp.113-128. extending
Roderick Chisholm Roderick Milton Chisholm (; November 27, 1916 – January 19, 1999) was an American philosopher known for his work on epistemology, metaphysics, free will, value theory, and the philosophy of perception. The '' Stanford Encyclopedia of Philoso ...
's (1946) notion of a "contrary-to-fact conditional" – 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 Sir Isaac Newton (25 December 1642 – 20 March 1726/27) was an English mathematician, physicist, astronomer, alchemist, theologian, and author (described in his time as a "natural philosopher"), widely recognised as one of the grea ...
and
Gottfried Leibniz Gottfried Wilhelm (von) Leibniz . ( – 14 November 1716) was a German polymath active as a mathematician, philosopher, scientist and diplomat. He is one of the most prominent figures in both the history of philosophy and the history of mathem ...
''had'' cooperated with each other, what would mathematics look like today?"). The study of counterfactual speculation has increasingly engaged the interest of scholars in a wide range of domains such as philosophy, psychology, cognitive psychology, history, political science, economics, social psychology, law, organizational theory, marketing, and epidemiology.


Semifactual

''Semifactual thought experiments'' – the term ''semifactual'' was coined by
Nelson Goodman Henry Nelson Goodman (7 August 1906 – 25 November 1998) was an American philosopher, known for his work on counterfactuals, mereology, the problem of induction, irrealism, and aesthetics. Life and career Goodman was born in Somerville, Mas ...
in 1947 – 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.


Predictive

The activity of
prediction A prediction (Latin ''præ-'', "before," and ''dicere'', "to say"), or forecast, is a statement about a future event or data. They are often, but not always, based upon experience or knowledge. There is no universal agreement about the exact ...
attempts to project the circumstances of the present into the future. According to David Sarewitz and Roger Pielke (1999, p123), scientific prediction takes two forms: # "The elucidation of invariant — and therefore predictive — principles of nature"; and # "
sing Singing is the act of creating musical sounds with the voice. A person who sings is called a singer, artist or vocalist (in jazz and/or popular music). Singers perform music (arias, recitatives, songs, etc.) that can be sung with or without ...
suites of observational data and sophisticated numerical models in an effort to foretell the behavior or evolution of complex phenomena". Although they perform different social and scientific functions, the only difference between the qualitatively identical activities of ''predicting'', ''forecasting,'' and ''nowcasting'' is the distance of the speculated future from the present moment occupied by the user. Whilst the activity of nowcasting, defined as "a detailed description of the current weather along with forecasts obtained by extrapolation up to 2 hours ahead", is essentially concerned with describing the current state of affairs, it is common practice to extend the term "to cover very-short-range forecasting up to 12 hours ahead" (Browning, 1982, p.ix).


Hindcasting

The activity of
hindcasting Backtesting is a term used in modeling to refer to testing a predictive model on historical data. Backtesting is a type of retrodiction, and a special type of cross-validation applied to previous time period(s). Financial analysis In a tradin ...
involves running a forecast model after an event has happened in order to test whether the model's
simulation A simulation is the imitation of the operation of a real-world process or system over time. Simulations require the use of Conceptual model, models; the model represents the key characteristics or behaviors of the selected system or proc ...
is valid.


Retrodiction

The activity of ''
retrodiction Retrodiction is the act of making a prediction about the past. It is also known as postdiction (but this should not be confused with the use of the term in criticisms of parapsychological research). Activity The activity of retrodiction (or po ...
'' (or ''postdiction'') involves moving backward in time, step-by-step, in as many stages as are considered necessary, from the present into the speculated past to establish the ultimate cause of a specific event (e.g.,
reverse engineering Reverse engineering (also known as backwards engineering or back engineering) is a process or method through which one attempts to understand through deductive reasoning how a previously made device, process, system, or piece of software accompli ...
and
forensics Forensic science, also known as criminalistics, is the application of science to criminal and civil laws, mainly—on the criminal side—during criminal investigation, as governed by the legal standards of admissible evidence and crimina ...
). Given that retrodiction is a process in which "past observations, events, add and data are used as evidence to infer the process(es) that produced them" and that
diagnosis Diagnosis is the identification of the nature and cause of a certain phenomenon. Diagnosis is used in many different disciplines, with variations in the use of logic, analytics, and experience, to determine " cause and effect". In systems engin ...
"involve going from visible effects such as symptoms, signs and the like to their prior causes", the essential balance between prediction and retrodiction could be characterized as: regardless of whether the
prognosis Prognosis (Greek: πρόγνωσις "fore-knowing, foreseeing") is a medical term for predicting the likely or expected development of a disease, including whether the signs and symptoms will improve or worsen (and how quickly) or remain stabl ...
is of the course of the disease in the absence of treatment, or of the application of a specific treatment regimen to a specific disorder in a particular patient.


Backcasting

The activity of ''
backcasting Backcasting is a planning method that starts with defining a desirable future and then works backwards to identify policies and programs that will connect that specified future to the present. The fundamentals of the method were outlined by John B. ...
'' – the term ''backcasting'' was coined by John Robinson in 1982 — involves establishing the description of a very definite and very specific future situation. It then involves an imaginary moving backward in time, step-by-step, in as many stages as are considered necessary, from the future to the present to reveal the mechanism through which that particular specified future could be attained from the present. Backcasting is not concerned with predicting the future: According to Jansen (1994, p. 503:


Fields

Thought experiments have been used in a variety of fields, including philosophy, law,
physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which r ...
, and mathematics. In philosophy they have been used at least since
classical antiquity Classical antiquity (also the classical era, classical period or classical age) is the period of cultural history between the 8th century BC and the 5th century AD centred on the Mediterranean Sea, comprising the interlocking civilizations of ...
, some pre-dating
Socrates Socrates (; ; –399 BC) was a Greek philosopher from Athens who is credited as the founder of Western philosophy and among the first moral philosophers of the ethical tradition of thought. An enigmatic figure, Socrates authored no te ...
. In law, they were well known to
Roman law Roman law is the law, legal system of ancient Rome, including the legal developments spanning over a thousand years of jurisprudence, from the Twelve Tables (c. 449 BC), to the ''Corpus Juris Civilis'' (AD 529) ordered by Eastern Roman emperor J ...
yers quoted in the Digest. In physics and other sciences, notable thought experiments date from the 19th and especially the 20th century, but examples can be found at least as early as
Galileo Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642) was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Commonly referred to as Galileo, his name was pronounced (, ). He was ...
.


Philosophy

In philosophy, a thought experiment typically presents an imagined scenario with the intention of eliciting an intuitive or reasoned response about the way things are in the thought experiment. (Philosophers might also supplement their thought experiments with theoretical reasoning designed to support the desired intuitive response.) The scenario will typically be designed to target a particular philosophical notion, such as morality, or the nature of the mind or linguistic reference. The response to the imagined scenario is supposed to tell us about the nature of that notion in any scenario, real or imagined. For example, a thought experiment might present a situation in which an agent intentionally kills an innocent for the benefit of others. Here, the relevant question is not whether the action is moral or not, but more broadly whether a moral theory is correct that says morality is determined solely by an action's consequences (See
Consequentialism In ethical philosophy, consequentialism is a class of normative, teleological ethical theories that holds that the consequences of one's conduct are the ultimate basis for judgment about the rightness or wrongness of that conduct. Thus, from a ...
).
John Searle John Rogers Searle (; born July 31, 1932) is an American philosopher widely noted for contributions to the philosophy of language, philosophy of mind, and social philosophy. He began teaching at UC Berkeley in 1959, and was Willis S. and Mario ...
imagines a man in a locked room who receives written sentences in Chinese, and returns written sentences in Chinese, according to a sophisticated instruction manual. Here, the relevant question is not whether or not the man understands Chinese, but more broadly, whether a functionalist theory of mind is correct. It is generally hoped that there is universal agreement about the intuitions that a thought experiment elicits. (Hence, in assessing their own thought experiments, philosophers may appeal to "what we should say," or some such locution.) A successful thought experiment will be one in which intuitions about it are widely shared. But often, philosophers differ in their intuitions about the scenario. Other philosophical uses of imagined scenarios arguably are thought experiments also. In one use of scenarios, philosophers might imagine persons in a particular situation (maybe ourselves), and ask what they would do. For example, in the
veil of ignorance The original position (OP), often referred to as the veil of ignorance, is a thought experiment used for reasoning about the principles that should structure a society based on mutual dependence. The phrases ''original position'' and ''veil of i ...
,
John Rawls John Bordley Rawls (; February 21, 1921 – November 24, 2002) was an American moral, legal and political philosopher in the liberal tradition. Rawls received both the Schock Prize for Logic and Philosophy and the National Humanities Medal in 1 ...
asks us to imagine a group of persons in a situation where they know nothing about themselves, and are charged with devising a social or political organization. The use of the
state of nature The state of nature, in moral and political philosophy, religion, social contract theories and international law, is the hypothetical life of people before societies came into existence. Philosophers of the state of nature theory deduce that ther ...
to imagine the origins of government, as by
Thomas Hobbes Thomas Hobbes ( ; 5/15 April 1588 – 4/14 December 1679) was an English philosopher, considered to be one of the founders of modern political philosophy. Hobbes is best known for his 1651 book ''Leviathan'', in which he expounds an influent ...
and
John Locke John Locke (; 29 August 1632 – 28 October 1704) was an English philosopher and physician, widely regarded as one of the most influential of Age of Enlightenment, Enlightenment thinkers and commonly known as the "father of liberalism ...
, may also be considered a thought experiment.
Søren Kierkegaard Søren Aabye Kierkegaard ( , , ; 5 May 1813 – 11 November 1855) was a Danish theologian, philosopher, poet, social critic, and religious author who is widely considered to be the first existentialist philosopher. He wrote critical texts on ...
explored the possible ethical and religious implications of
Abraham Abraham, ; ar, , , name=, group= (originally Abram) is the common Hebrew patriarch of the Abrahamic religions, including Judaism, Christianity, and Islam. In Judaism, he is the founding father of the special relationship between the Jew ...
's
binding of Isaac The Binding of Isaac ( he, , ), or simply "The Binding" (, ), is a story from Genesis 22 of the Hebrew Bible. In the biblical narrative, God tells Abraham to sacrifice his son, Isaac, on Moriah. As Abraham begins to comply, having bound Isaa ...
in ''
Fear and Trembling ''Fear and Trembling'' (original Danish title: ''Frygt og Bæven'') is a philosophical work by Søren Kierkegaard, published in 1843 under the pseudonym ''Johannes de silentio'' (Latin for ''John of the Silence''). The title is a reference to ...
''. Similarly,
Friedrich Nietzsche Friedrich Wilhelm Nietzsche (; or ; 15 October 1844 – 25 August 1900) was a German philosopher, prose poet, cultural critic, philologist, and composer whose work has exerted a profound influence on contemporary philosophy. He began his ...
, in ''
On the Genealogy of Morals ''On the Genealogy of Morality: A Polemic'' (german: Zur Genealogie der Moral: Eine Streitschrift) is an 1887 book by German philosopher Friedrich Nietzsche. It consists of a preface and three interrelated treatises ('Abhandlungen' in German) that ...
'', speculated about the historical development of Judeo-Christian morality, with the intent of questioning its legitimacy. An early written thought experiment was
Plato Plato ( ; grc-gre, Πλάτων ; 428/427 or 424/423 – 348/347 BC) was a Greek philosopher born in Athens during the Classical period in Ancient Greece. He founded the Platonist school of thought and the Academy, the first institution ...
's
allegory of the cave The Allegory of the Cave, or Plato's Cave, is an allegory presented by the Ancient Greece, Greek philosopher Plato in his work ''Republic (Plato), Republic'' (514a–520a) to compare "the effect of education (Wiktionary:παιδεία, παιδ ...
. Another historic thought experiment was
Avicenna Ibn Sina ( fa, ابن سینا; 980 – June 1037 CE), commonly known in the West as Avicenna (), was a Persian polymath who is regarded as one of the most significant physicians, astronomers, philosophers, and writers of the Islamic G ...
's "
Floating Man Floating man is the proper translation of the verb "yahwā in al-Nafs," which means "to fall down." Flying man is another term used cohesively to describe a floating man. According to Ibn Sina, it is considered a thought experience to determine ...
" 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 In philosophy of self, self-awareness is the experience of one's own personality or individuality. It is not to be confused with consciousness in the sense of qualia. While consciousness is being aware of one's environment and body and lifesty ...
and
self-consciousness Self-consciousness is a heightened sense of awareness of oneself. It is not to be confused with consciousness in the sense of qualia. Historically, "self-consciousness" was synonymous with " self-awareness", referring to a state of awareness that ...
, and the substantiality of the
soul In many religious and philosophical traditions, there is a belief that a soul is "the immaterial aspect or essence of a human being". Etymology The Modern English noun ''soul'' is derived from Old English ''sāwol, sāwel''. The earliest attes ...
.


Science

Scientists tend to use thought experiments as imaginary, "proxy" experiments prior to a real, "physical" experiment (
Ernst Mach Ernst Waldfried Josef Wenzel Mach ( , ; 18 February 1838 – 19 February 1916) was a Moravian-born Austrian physicist and philosopher, who contributed to the physics of shock waves. The ratio of one's speed to that of sound is named the Mach ...
always argued that these ''gedankenexperiments'' were "a necessary precondition for physical experiment"). In these cases, the result of the "proxy" experiment will often be so clear that there will be no need to conduct a physical experiment at all. Scientists also use thought experiments when particular physical experiments are impossible to conduct (
Carl Gustav Hempel Carl Gustav "Peter" Hempel (January 8, 1905 – November 9, 1997) was a German writer, philosopher, logician, and epistemologist. He was a major figure in logical empiricism, a 20th-century movement in the philosophy of science. He is espec ...
labeled these sorts of experiment "''theoretical experiments-in-imagination''"), such as Einstein's thought experiment of chasing a light beam, leading to
special relativity In physics, the special theory of relativity, or special relativity for short, is a scientific theory regarding the relationship between space and time. In Albert Einstein's original treatment, the theory is based on two postulates: # The laws o ...
. This is a unique use of a scientific thought experiment, in that it was never carried out, but led to a successful theory, proven by other empirical means.


Properties

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


Possibility

In many thought experiments, the scenario would be nomologically possible, or possible according to the laws of nature. John Searle's Chinese room is
nomological In philosophy, nomology refers to a "science of laws" based on the theory that it is possible to elaborate descriptions dedicated not to particular aspects of reality but inspired by a scientific vision of universal validity expressed by scientific ...
ly possible. Some thought experiments present scenarios that are not nomologically possible. In his
Twin Earth thought experiment Twin Earth is a thought experiment proposed by philosopher Hilary Putnam in his papers "Meaning and Reference" (1973) and "The Meaning of 'Meaning (1975). It is meant to serve as an illustration of his argument for semantic externalism, or the ...
,
Hilary Putnam Hilary Whitehall Putnam (; July 31, 1926 – March 13, 2016) was an American philosopher, mathematician, and computer scientist, and a major figure in analytic philosophy in the second half of the 20th century. He made significant contributions ...
asks us to imagine a scenario in which there is a substance with all of the observable properties of water (e.g., taste, color, boiling point), but is chemically different from water. It has been argued that this thought experiment is not nomologically possible, although it may be possible in some other sense, such as
metaphysical possibility Subjunctive possibility (also called alethic possibility) is a form of modality studied in modal logic. Subjunctive possibilities are the sorts of possibilities considered when conceiving counterfactual situations; subjunctive modalities are modal ...
. 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 John Chalmers (; born 20 April 1966) is an Australian philosopher and cognitive scientist specializing in the areas of philosophy of mind and philosophy of language. He is a professor of philosophy and neural science at New York Universi ...
says that we can imagine that there are
zombies A zombie (Haitian French: , ht, zonbi) is a mythological undead corporeal revenant created through the reanimation of a corpse. Zombies are most commonly found in horror and fantasy genre works. The term comes from Haitian folklore, in wh ...
, or persons who are physically identical to us in every way but who lack consciousness. This is supposed to show that
physicalism In philosophy, physicalism is the metaphysical thesis that "everything is physical", that there is "nothing over and above" the physical, or that everything supervenes on the physical. Physicalism is a form of ontological monism—a "one substanc ...
is false. However, some argue that zombies are inconceivable: we can no more imagine a zombie than we can imagine that 1+1=3. Others have claimed that the conceivability of a scenario may not entail its possibility.


Causal reasoning

The first characteristic pattern that thought experiments display is their orientation in time. They are either: *''Antefactual speculations'': experiments that speculate about what might have happened prior to a specific, designated event, or *''Postfactual speculations'': experiments that speculate about what may happen subsequent to (or consequent upon) a specific, designated event. The second characteristic pattern is their movement in time in relation to "the present moment standpoint" of the individual performing the experiment; namely, in terms of: * Their ''temporal direction'': are they past-oriented or future-oriented? * Their ''temporal sense'': **(a) in the case of past-oriented thought experiments, are they examining the consequences of temporal "movement" from the present to the past, or from the past to the present? or, **(b) in the case of future-oriented thought experiments, are they examining the consequences of temporal "movement" from the present to the future, or from the future to the present?


Relation to real experiments

The relation to real experiments can be quite complex, as can be seen again from an example going back to Albert Einstein. In 1935, with two coworkers, he published a paper on a newly created subject called later the EPR effect (
EPR paradox EPR may refer to: Science and technology * EPR (nuclear reactor), European Pressurised-Water Reactor * EPR paradox (Einstein–Podolsky–Rosen paradox), in physics * Earth potential rise, in electrical engineering * East Pacific Rise, a mid-oc ...
). 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 Henrik David Bohr (; 7 October 1885 – 18 November 1962) was a Danish physicist who made foundational contributions to understanding atomic structure and quantum theory, for which he received the Nobel Prize in Physics in 1922. B ...
asserted a refutation of Einstein's analysis immediately, and his view prevailed. After some decades, it was asserted that feasible experiments could prove the error of the EPR paper. These experiments tested the Bell inequalities published in 1964 in a purely theoretical paper. The above-mentioned EPR philosophical starting assumptions were considered to be falsified by the empirical fact (e.g. by the optical ''real experiments'' of
Alain Aspect Alain Aspect (; born 15 June 1947) is a French physicist noted for his experimental work on quantum entanglement. Aspect was awarded the 2022 Nobel Prize in Physics, jointly with John Clauser and Anton Zeilinger, "for experiments with entangl ...
). Thus ''thought experiments'' belong to a theoretical discipline, usually to
theoretical physics Theoretical physics is a branch of physics that employs mathematical models and abstractions of physical objects and systems to rationalize, explain and predict natural phenomena. This is in contrast to experimental physics, which uses experim ...
, but often to
theoretical philosophy The modern division of philosophy into theoretical philosophy and practical philosophyImmanuel Kant, ''Lectures on Ethics'', Cambridge University Press, 2001, p. 41 ("On Universal Practical Philosophy"). Original text: Immanuel Kant, ''Kant’s Ges ...
. In any case, it must be distinguished from a real experiment, which belongs naturally to the experimental discipline and has "the final decision on ''true'' or ''not true''", at least in physics.


Interactivity

Thought experiments can also be interactive where the author invites people into his thought process through providing alternative paths with alternative outcomes within the narrative, or through interaction with a programmed machine, like a computer program. Thanks to the advent of the Internet, the digital space has lent itself as a new medium for a new kind of thought experiments. The philosophical work of
Stefano Gualeni Stefano Gualeni, Ph.D., is an Italian philosopher and game designer who created videogames such as '' Tony Tough and the Night of Roasted Moths,'' '' Gua-Le-Ni; or, The Horrendous Parade,'' and '' Something Something Soup Something''. Gualeni i ...
, for example, focuses on the use of virtual worlds to materialize thought experiments and to playfully negotiate philosophical ideas. His arguments were originally presented in his book ''Virtual Worlds as Philosophical Tools''. Gualeni's argument is that the history of philosophy has, until recently, merely been the history of written thought, and digital media can complement and enrich the limited and almost exclusively linguistic approach to philosophical thought. He considers virtual worlds to be philosophically viable and advantageous in contexts like those of thought experiments, when the recipients of a certain philosophical notion or perspective are expected to objectively test and evaluate different possible courses of action, or in cases where they are confronted with interrogatives concerning non-actual or non-human phenomenologies.


Examples


Humanities

*
Doomsday argument The Doomsday Argument (DA), or Carter catastrophe, is a probabilistic argument that claims to predict the future population of the human species, based on an estimation of the number of humans born to date. The Doomsday argument was originally ...
(
anthropic principle The anthropic principle, also known as the "observation selection effect", is the hypothesis, first proposed in 1957 by Robert Dicke, that there is a restrictive lower bound on how statistically probable our observations of the universe are, beca ...
) * The Lady, or the Tiger? (human nature)While the problem presented in this short story's scenario is not unique, it is extremely unusual. Most thought experiments are intentionally (or, even, sometimes unintentionally) skewed towards the inevitable production of a particular solution to the problem posed; and this happens because of the way that the problem and the scenario are framed in the first place. In the case of ''The Lady, or the Tiger?'', the way that the story unfolds is so "end-neutral" that, at the finish, there is no "correct" solution to the problem. Therefore, all that one can do is to offer one's own innermost thoughts on how the account of human nature that has been presented might unfold – according to one's own experience of human nature – which is, obviously, the purpose of the entire exercise. The extent to which the story can provoke such an extremely wide range of (otherwise equipollent) predictions of the participants' subsequent behaviour is one of the reasons the story has been so popular over time. * The beer question (
U.S. politics The politics of the United States function within a framework of a constitutional federal republic and presidential system, with three distinct branches that Separation of powers, share powers. These are: the United States Congress, U.S. Congre ...
)


Physics

*
Bell's spaceship paradox Bell's spaceship paradox is a thought experiment in special relativity. It was designed by E. Dewan and M. Beran in 1959 and became more widely known when J. S. Bell included a modified version.J. S. Bell: ''How to teach special relativity'', Prog ...
(
special relativity In physics, the special theory of relativity, or special relativity for short, is a scientific theory regarding the relationship between space and time. In Albert Einstein's original treatment, the theory is based on two postulates: # The laws o ...
) *
Brownian ratchet In the philosophy of thermal and statistical physics, the Brownian ratchet or Feynman–Smoluchowski ratchet is an apparent perpetual motion machine of the second kind (converting thermal energy into mechanical work), first analysed in 1912 as a ...
(
Richard Feynman Richard Phillips Feynman (; May 11, 1918 – February 15, 1988) was an American theoretical physicist, known for his work in the path integral formulation of quantum mechanics, the theory of quantum electrodynamics, the physics of the superflu ...
's "
perpetual motion Perpetual motion is the motion of bodies that continues forever in an unperturbed system. A perpetual motion machine is a hypothetical machine that can do work infinitely without an external energy source. This kind of machine is impossible, a ...
" machine that does not violate the second law and does no work at thermal equilibrium) *
Bucket argument Isaac Newton's rotating bucket argument (also known as Newton's bucket) 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. It is one of ...
– argues that space is absolute, not relational *
Dyson sphere A Dyson sphere is a hypothetical megastructure that completely encompasses a star and captures a large percentage of its solar power output. The concept is a thought experiment that attempts to explain how a spacefaring civilization would meet ...
* Einstein's box * Elitzur–Vaidman bomb-tester (
quantum mechanics Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, ...
) *
EPR paradox EPR may refer to: Science and technology * EPR (nuclear reactor), European Pressurised-Water Reactor * EPR paradox (Einstein–Podolsky–Rosen paradox), in physics * Earth potential rise, in electrical engineering * East Pacific Rise, a mid-oc ...
(
quantum mechanics Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, ...
) (forms of this have been performed) *
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. The question of how such a device would turn was the ...
(classical mechanics) *
Galileo's Leaning Tower of Pisa experiment Between 1589 and 1592, the Italian scientist Galileo Galilei (then professor of mathematics at the University of Pisa) is said to have dropped two spheres of different masses from the Leaning Tower of Pisa to demonstrate that their time of descent ...
(rebuttal of Aristotelian Gravity) *
Galileo's ship Galileo's ship refers to two physics experiments, a thought experiment and an actual experiment, by Galileo Galilei, the 16th and 17th century physicist and astronomer. The experiments were created to argue the idea of a rotating Earth as opposed t ...
(classical relativity principle) 1632 * GHZ experiment (
quantum mechanics Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, ...
) *
Heisenberg's microscope Heisenberg's microscope is a thought experiment proposed by Werner Heisenberg that has served as the nucleus of some commonly held ideas about quantum mechanics. In particular, it provides an argument for the uncertainty principle on the basis of t ...
(
quantum mechanics Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, ...
) * Kepler's Dream (change of point of view as support for the Copernican hypothesis) *
Ladder paradox The ladder paradox (or barn-pole paradox) is a thought experiment in special relativity. It involves a ladder, parallel to the ground, travelling horizontally at relativistic speed (near the speed of light) and therefore undergoing a Lorentz lengt ...
(
special relativity In physics, the special theory of relativity, or special relativity for short, is a scientific theory regarding the relationship between space and time. In Albert Einstein's original treatment, the theory is based on two postulates: # The laws o ...
) *
Laplace's demon In the history of science, Laplace's demon was a notable published articulation of causal determinism on a scientific basis by Pierre-Simon Laplace in 1814. According to determinism, if someone (the demon) knows the precise location and moment ...
*
Maxwell's demon Maxwell's demon is a thought experiment that would hypothetically violate the second law of thermodynamics. It was proposed by the physicist James Clerk Maxwell in 1867. In his first letter Maxwell called the demon a "finite being", while the ' ...
(
thermodynamics Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by the four laws of the ...
) 1871 *
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 vel ...
*
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. It appeared in his posthumously published 1728 work ''De mundi systemate'' (als ...
(
Newton's laws of motion Newton's laws of motion are three basic laws of classical mechanics that describe the relationship between the motion of an object and the forces acting on it. These laws can be paraphrased as follows: # A body remains at rest, or in moti ...
) * Popper's experiment (
quantum mechanics Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, ...
) * Quantum pseudo telepathy (
quantum mechanics Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, ...
) *
Quantum suicide and immortality Quantum suicide is a thought experiment in quantum mechanics and the philosophy of physics. Purportedly, it can falsify any interpretation of quantum mechanics other than the Everett many-worlds interpretation by means of a variation of the Schrö ...
(
quantum mechanics Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, ...
) *
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. The statement is that a pa ...
(
quantum mechanics Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, ...
) *
Schrödinger's cat In quantum mechanics, Schrödinger's cat is a thought experiment that illustrates a paradox of quantum superposition. In the thought experiment, a hypothetical cat may be considered simultaneously both alive and dead, while it is unobserved in ...
(
quantum mechanics Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, ...
) *
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. These claims were not widely accepted prior to ...
(
general relativity General relativity, also known as the general theory of relativity and Einstein's theory of gravity, is the geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of gravitation in modern physics ...
) * The Monkey and the Hunter (
gravitation In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stron ...
) *
Twin paradox In physics, the twin paradox is a thought experiment in special relativity involving identical twins, one of whom makes a journey into space in a high-speed rocket and returns home to find that the twin who remained on Earth has aged more. Thi ...
(
special relativity In physics, the special theory of relativity, or special relativity for short, is a scientific theory regarding the relationship between space and time. In Albert Einstein's original treatment, the theory is based on two postulates: # The laws o ...
) *
Wheeler's delayed choice experiment Wheeler's delayed-choice experiment describes a family of thought experiments in quantum physics proposed by John Archibald Wheeler, with the most prominent among them appearing in 1978 and 1984. These experiments are attempts to decide whether ...
(
quantum mechanics Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, ...
) *
Wigner's friend Wigner's friend is a thought experiment in theoretical quantum physics, first conceived by the physicist Eugene Wigner in 1961, Reprinted in and further developed by David Deutsch in 1985. The scenario involves an indirect observation of a Measure ...
(
quantum mechanics Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, ...
)


Philosophy

* Artificial brain *
Avicenna Ibn Sina ( fa, ابن سینا; 980 – June 1037 CE), commonly known in the West as Avicenna (), was a Persian polymath who is regarded as one of the most significant physicians, astronomers, philosophers, and writers of the Islamic G ...
's
Floating Man Floating man is the proper translation of the verb "yahwā in al-Nafs," which means "to fall down." Flying man is another term used cohesively to describe a floating man. According to Ibn Sina, it is considered a thought experience to determine ...
* Beetle in a box * '' Bellum omnium contra omnes'' * Big Book (ethics) * Brain-in-a-vat (
epistemology Epistemology (; ), or the theory of knowledge, is the branch of philosophy concerned with knowledge. Epistemology is considered a major subfield of philosophy, along with other major subfields such as ethics, logic, and metaphysics. Episte ...
,
philosophy of mind Philosophy of mind is a branch of philosophy that studies the ontology and nature of the mind and its relationship with the body. The mind–body problem is a paradigmatic issue in philosophy of mind, although a number of other issues are addre ...
) *
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 In philosophy of mind, qualia ...
*
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 (donkey) that is equally hungry and thirsty is placed precisely midway between a stack of hay and a ...
*
Changing places ''Changing Places'' (1975) is the first "campus novel" by British novelist David Lodge. The subtitle is "A Tale of Two Campuses", and thus both the title and subtitle are literary allusions to Charles Dickens' ''A Tale of Two Cities''. It is t ...
( reflexive monism,
philosophy of mind Philosophy of mind is a branch of philosophy that studies the ontology and nature of the mind and its relationship with the body. The mind–body problem is a paradigmatic issue in philosophy of mind, although a number of other issues are addre ...
) * China brain (
physicalism In philosophy, physicalism is the metaphysical thesis that "everything is physical", that there is "nothing over and above" the physical, or that everything supervenes on the physical. Physicalism is a form of ontological monism—a "one substanc ...
,
philosophy of mind Philosophy of mind is a branch of philosophy that studies the ontology and nature of the mind and its relationship with the body. The mind–body problem is a paradigmatic issue in philosophy of mind, although a number of other issues are addre ...
) * Chinese room (
philosophy of mind Philosophy of mind is a branch of philosophy that studies the ontology and nature of the mind and its relationship with the body. The mind–body problem is a paradigmatic issue in philosophy of mind, although a number of other issues are addre ...
,
artificial intelligence Artificial intelligence (AI) is intelligence—perceiving, synthesizing, and inferring information—demonstrated by machines, as opposed to intelligence displayed by animals and humans. Example tasks in which this is done include speech re ...
, cognitive science) *
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 ...
* Condillac's Statue (
epistemology Epistemology (; ), or the theory of knowledge, is the branch of philosophy concerned with knowledge. Epistemology is considered a major subfield of philosophy, along with other major subfields such as ethics, logic, and metaphysics. Episte ...
) *
Experience machine The experience machine or pleasure machine is a thought experiment put forward by philosopher Robert Nozick in his 1974 book '' Anarchy, State, and Utopia''. It is an attempt to refute ethical hedonism by imagining a choice between everyday real ...
(ethics) *
Gettier problem The Gettier problem, in the field of epistemology, is a landmark philosophical problem concerning the understanding of descriptive knowledge. Attributed to American philosopher Edmund Gettier, Gettier-type counterexamples (called "Gettier-cases") ...
(epistemology) * Ḥayy ibn Yaqẓān (epistemology) *
Hilary Putnam Hilary Whitehall Putnam (; July 31, 1926 – March 13, 2016) was an American philosopher, mathematician, and computer scientist, and a major figure in analytic philosophy in the second half of the 20th century. He made significant contributions ...
's
Twin Earth thought experiment Twin Earth is a thought experiment proposed by philosopher Hilary Putnam in his papers "Meaning and Reference" (1973) and "The Meaning of 'Meaning (1975). It is meant to serve as an illustration of his argument for semantic externalism, or the ...
in the
philosophy of language In analytic philosophy, philosophy of language investigates the nature of language and the relations between language, language users, and the world. Investigations may include inquiry into the nature of meaning, intentionality, reference, ...
and
philosophy of mind Philosophy of mind is a branch of philosophy that studies the ontology and nature of the mind and its relationship with the body. The mind–body problem is a paradigmatic issue in philosophy of mind, although a number of other issues are addre ...
*
If a tree falls in a forest "If a tree falls in a forest and no one is around to hear it, does it make a sound?" is a philosophical thought experiment that raises questions regarding observation and perception. History While the origin of the phrase is sometimes mistakenl ...
*
Inverted spectrum The inverted spectrum is the hypothetical concept, pertaining to the philosophy of color, of two people sharing their color vocabulary and discriminations, although the colors one sees—one's qualia—are systematically different from the colors t ...
*
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 The knowledge argument (also known as Mary's room or Mary the super-scientist) is a philosophical thought experiment proposed by Frank Jackson in his article "Epiphenomenal Qualia" (1982) and extended in "What Mary Didn't Know" (1986). The experim ...
(
philosophy of mind Philosophy of mind is a branch of philosophy that studies the ontology and nature of the mind and its relationship with the body. The mind–body problem is a paradigmatic issue in philosophy of mind, although a number of other issues are addre ...
) *
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 referred to in John Locke's ''An Essay Concerning Human Understanding'' (1689). The p ...
(admittedly, this oscillated between empirical and a-priori assessment) *
Newcomb's paradox In philosophy and mathematics, Newcomb's paradox, also known as Newcomb's problem, is a thought experiment involving a game between two players, one of whom is able to predict the future. Newcomb's paradox was created by William Newcomb of the ...
*
Original position The original position (OP), often referred to as the veil of ignorance, is a thought experiment used for reasoning about the principles that should structure a society based on mutual dependence. The phrases ''original position'' and ''veil of i ...
(politics) *
Philosophical zombie A philosophical zombie or p-zombie argument is a thought experiment in philosophy of mind that imagines a hypothetical being that is physically identical to and indistinguishable from a normal person but does not have consciousness, conscious ex ...
(
philosophy of mind Philosophy of mind is a branch of philosophy that studies the ontology and nature of the mind and its relationship with the body. The mind–body problem is a paradigmatic issue in philosophy of mind, although a number of other issues are addre ...
,
artificial intelligence Artificial intelligence (AI) is intelligence—perceiving, synthesizing, and inferring information—demonstrated by machines, as opposed to intelligence displayed by animals and humans. Example tasks in which this is done include speech re ...
, cognitive science) *
Plank of Carneades In ethics, the plank of Carneades is a thought experiment first proposed by the Academic Skeptic philosopher Carneades of Cyrene. It explores the concept of self-defense in relation to murder. In the thought experiment, there are two shipwrecked ...
* Roko's basilisk *
Ship of Theseus The Ship of Theseus is a thought experiment about whether an object that has had all of its original components replaced remains the same object. According to legend, Theseus, the mythical Greek founder-king of Athens, had rescued the children of ...
, The (concept of
identity Identity may refer to: * Identity document * Identity (philosophy) * Identity (social science) * Identity (mathematics) Arts and entertainment Film and television * ''Identity'' (1987 film), an Iranian film * ''Identity'' (2003 film), ...
) * Shoemaker's "''Time Without Change''" (metaphysics) * Simulated reality (philosophy,
computer science Computer science is the study of computation, automation, and information. Computer science spans theoretical disciplines (such as algorithms, theory of computation, information theory, and automation) to Applied science, practical discipli ...
, cognitive science) *
Social contract In moral and political philosophy Political philosophy or political theory is the philosophical study of government, addressing questions about the nature, scope, and legitimacy of public agents and institutions and the relationships betw ...
theories * Survival lottery, The (ethics) *
Swamp man Donald Herbert Davidson (March 6, 1917 – August 30, 2003) was an American philosopher. He served as Slusser Professor of Philosophy at the University of California, Berkeley, from 1981 to 2003 after having also held teaching appointments at ...
(
personal identity Personal identity is the unique numerical identity of a person over time. Discussions regarding personal identity typically aim to determine the necessary and sufficient conditions under which a person at one time and a person at another time can ...
,
philosophy of mind Philosophy of mind is a branch of philosophy that studies the ontology and nature of the mind and its relationship with the body. The mind–body problem is a paradigmatic issue in philosophy of mind, although a number of other issues are addre ...
) *
Teleportation Teleportation is the hypothetical transfer of matter or energy from one point to another without traversing the physical space between them. It is a common subject in science fiction literature and in other popular culture. Teleportation is oft ...
(metaphysics) * The Transparent eyeball * The Violinist (ethics) *
Ticking time bomb scenario The ticking time bomb scenario is a thought experiment that has been used in the ethics debate over whether interrogational torture can ever be justified. The scenario can be formulated as follows: Suppose that a person with knowledge of an immi ...
(ethics) *
Trolley problem The trolley problem is a series of thought experiments in ethics and psychology, involving stylized ethical dilemmas of whether to sacrifice one person to save a larger number. The series usually begins with a scenario in which a runaway tram or ...
(ethics) * Utility monster (ethics) *
Zeno's paradoxes Zeno's paradoxes are a set of philosophical problems generally thought to have been devised by Greek philosopher Zeno of Elea (c. 490–430 BC) to support Parmenides' doctrine that contrary to the evidence of one's senses, the belief in pluralit ...
(classical Greek problems of the infinite)


Mathematics

* Balls and vase problem (infinity and cardinality) *
Gabriel's Horn Gabriel's horn (also called Torricelli's trumpet) is a particular geometry, geometric figure that has infinite surface area but finite volume. The name refers to the Christian tradition where the archangel Gabriel blows the horn to announce Last ...
(infinity) *
Hilbert's paradox of the Grand Hotel Hilbert's paradox of the Grand Hotel (colloquial: Infinite Hotel Paradox or Hilbert's Hotel) is a thought experiment which illustrates a counterintuitive property of infinite sets. It is demonstrated that a fully occupied hotel with infinitely m ...
(infinity) *
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 any given text, such as the complete works of William Shakespeare. In fact, the monkey would ...
(probability) *
Lottery paradox The lottery paradoxKyburg, H. E. (1961). ''Probability and the Logic of Rational Belief'', Middletown, CT: Wesleyan University Press, p. 197. arises from Henry E. Kyburg Jr. considering a fair 1,000-ticket lottery that has exactly one winning ti ...
(probability) * Sleeping beauty paradox (probability)


Biology

*
Levinthal paradox Levinthal's paradox is a thought experiment, also constituting a self-reference in the theory of protein folding. In 1969, Cyrus Levinthal noted that, because of the very large number of degrees of freedom in an unfolded polypeptide chain, the m ...
*
Rotating locomotion in living systems Rotation, or spin, is the circular movement of an object around a '' central axis''. A two-dimensional rotating object has only one possible central axis and can rotate in either a clockwise or counterclockwise direction. A three-dimensional ...


Computer science

* Braitenberg vehicles (robotics, neural control and sensing systems) (some have been built) * Dining Philosophers (
computer science Computer science is the study of computation, automation, and information. Computer science spans theoretical disciplines (such as algorithms, theory of computation, information theory, and automation) to Applied science, practical discipli ...
) *
Halting problem In computability theory, the halting problem is the problem of determining, from a description of an arbitrary computer program and an input, whether the program will finish running, or continue to run forever. Alan Turing proved in 1936 that a g ...
(limits of computability) *
Turing machine A Turing machine is a mathematical model of computation describing an abstract machine that manipulates symbols on a strip of tape according to a table of rules. Despite the model's simplicity, it is capable of implementing any computer algori ...
(limits of computability) *
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. In the experiment, two generals are only able t ...


Economics

*
Broken window fallacy The parable of the broken window was introduced by French economist Frédéric Bastiat in his 1850 essay " That Which We See and That Which We Do Not See" ("") to illustrate why destruction, and the money spent to recover from destruction, is not ...
(
law of unintended consequences In the social sciences, unintended consequences (sometimes unanticipated consequences or unforeseen consequences) are outcomes of a purposeful action that are not intended or foreseen. The term was popularised in the twentieth century by Ameri ...
,
opportunity cost In microeconomic theory, the opportunity cost of a particular activity is the value or benefit given up by engaging in that activity, relative to engaging in an alternative activity. More effective it means if you chose one activity (for example ...
) * Laffer Curve


See also

*
Alternate history Alternate history (also alternative history, althist, AH) is a genre of speculative fiction of stories in which one or more historical events occur and are resolved differently than in real life. As conjecture based upon historical fact, altern ...
*
Aporia In philosophy, an aporia ( grc, ᾰ̓πορῐ́ᾱ, aporíā, literally: "lacking passage", also: "impasse", "difficulty in passage", "puzzlement") is a conundrum or state of puzzlement. In rhetoric, it is a declaration of doubt, made for rh ...
*
Black box In science, computing, and engineering, a black box is a system which can be viewed in terms of its inputs and outputs (or transfer characteristics), without any knowledge of its internal workings. Its implementation is "opaque" (black). The te ...
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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 In philosophy of mind, qualia ...
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Ding an sich In Kantian philosophy, the thing-in-itself (german: Ding an sich) is the status of objects as they are, independent of representation and observation. The concept of the thing-in-itself was introduced by the German philosopher Immanuel Kant, and ...
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Einstein's thought experiments A hallmark of Albert Einstein's career was his use of visualized thought experiments (german: Gedankenexperiment) as a fundamental tool for understanding physical issues and for elucidating his concepts to others. Einstein's thought experiments too ...
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Futures studies Futures studies, futures research, futurism or futurology is the systematic, interdisciplinary and holistic study of social and technological advancement, and other environmental trends, often for the purpose of exploring how people will li ...
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Futures techniques Futures techniques used in the multi-disciplinary field of futurology by futurists in Americas and Australasia, and futurology by futurologists in EU, include a diverse range of forecasting methods, including anticipatory thinking, backcasting, si ...
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Heuristic A heuristic (; ), or heuristic technique, is any approach to problem solving or self-discovery that employs a practical method that is not guaranteed to be optimal, perfect, or rational, but is nevertheless sufficient for reaching an immediate, ...
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Intuition pump An intuition pump is a thought experiment structured to allow the thinker to use their intuition to develop an answer to a problem. In Dennett's work The term was coined by Daniel Dennett. In ''Consciousness Explained'', he uses the term to desc ...
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Mathematical proof A mathematical proof is an inferential argument for a mathematical statement, showing that the stated assumptions logically guarantee the conclusion. The argument may use other previously established statements, such as theorems; but every proo ...
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N-universes The n-universes are a conceptual tool introduced by philosopher Paul Franceschi. They consist of simplified models of universes which are reduced to their essential components, in order to facilitate the associated reasoning. In the study of thoug ...
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Possible world A possible world is a complete and consistent way the world is or could have been. Possible worlds are widely used as a formal device in logic, philosophy, and linguistics in order to provide a semantics for intensional logic, intensional and mod ...
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Scenario planning Scenario planning, scenario thinking, scenario analysis, scenario prediction and the scenario method all describe a strategic planning method that some organizations use to make flexible long-term plans. It is in large part an adaptation and gener ...
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Scenario test Scenario testing is a software testing activity that uses scenarios: hypothetical stories to help the tester work through a complex problem or test system. The ideal scenario test is a credible, complex, compelling or motivating story; the outcome ...
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Theoretical physics Theoretical physics is a branch of physics that employs mathematical models and abstractions of physical objects and systems to rationalize, explain and predict natural phenomena. This is in contrast to experimental physics, which uses experim ...


Notes


References


Further reading


Brendal, Elke, "Intuition Pumps and the Proper Use of Thought Experiments", ''Dialectica'', Vol.58, No.1, (March 2004, pp.89–108.
* Ćorić, Dragana (2020), "The Importance of Thought Experiments", ''Journal of Eastern-European Criminal Law'', Vol.2020, No.1, (2020), pp. 127–135.
Cucic, D.A. & Nikolic, A.S., "A short insight about thought experiment in modern physics", ''6th International Conference of the Balkan Physical Union BPU6'', Istanbul – Turkey, 2006.
* 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. * 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. * Kuhn, T., "A Function for Thought Experiments", in ''The Essential Tension'' (Chicago: University of Chicago Press, 1979), pp. 240–265. * 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. * Stuart, M. T., Fehige, Y. and Brown, J. R. (2018). The Routledge Companion to Thought Experiments. London: Routledge. * Witt-Hansen, J., "H.C. Ørsted, Immanuel Kant and the Thought Experiment", ''Danish Yearbook of Philosophy'', Vol.13, (1976), pp. 48–65.


Bibliography

* Adams, Scott, ''God's Debris: A Thought Experiment'', Andrews McMeel Publishing, (USA), 2001 * 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. & Hawthorne, J., ''Conceivability and Possibility'', Oxford University Press, (Oxford), 2002. * Gendler, T.S., ''Thought Experiment: On the Powers and Limits of Imaginary Cases'', Garland, (New York), 2000. * 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. , ''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.
Yeates, L.B., ''Thought Experimentation: A Cognitive Approach'', Graduate Diploma in Arts (By Research) Dissertation, University of New South Wales, 2004.


External links

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Short essay by S. Abbas Raza of '' 3 Quarks Daily''
Thought experiment generator
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