Thought experiment


A thought experiment considers a hypothesis, theory, or principle for the purpose of thinking through its consequences.
Johann Witt-Hansen established that Hans Christian Ørsted was the first to use the German term Gedankenexperiment circa 1812. Ørsted was also the first to use the equivalent term Gedankenversuch in 1820.
Much later, Ernst Mach used the term Gedankenexperiment 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 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. 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.
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, illustrating quantum indeterminacy through the manipulation of a perfectly sealed environment and a tiny bit of radioactive substance, and Maxwell's demon, which attempts to demonstrate the ability of a hypothetical finite being to violate the 2nd law of thermodynamics.

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.
Perhaps the key experiment in the history of modern science is Galileo'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 a logical demonstration, using the 'thought experiment' technique. The 'experiment' is described by Galileo in Discorsi e dimostrazioni matematiche 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. Instead, many philosophers prefer to consider 'Thought Experiments' to be merely the use of a hypothetical scenario to help understand the way things are.

Uses

Thought experiments, which are well-structured, well-defined hypothetical questions that employ subjunctive reasoning – "What might happen if... " – have been used to pose questions in philosophy at least since Greek antiquity, some pre-dating Socrates. In physics and other sciences many thought experiments date from the 19th and especially the 20th Century, but examples can be found at least as early as Galileo.
In thought experiments we gain new information by rearranging or reorganizing already known empirical data in a new way and drawing new inferences from them or by looking at these data from a different and unusual perspective. In Galileo's thought experiment, for example, the rearrangement of empirical experience consists in the original idea of combining bodies of different weight.
Thought experiments have been used in philosophy, physics, and other fields. 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:
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:
Generally speaking, there are seven types of thought experiments in which one reasons from causes to effects, or effects to causes:

Prefactual

Prefactual 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 thought experiments — the term counterfactual was coined by Nelson Goodman in 1947, extending Roderick Chisholm's 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?".
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 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?.
Semifactual speculations are an important part of clinical medicine.

Prediction

The activity of prediction attempts to project the circumstances of the present into the future. According to David Sarewitz and Roger Pielke, scientific prediction takes two forms:
  1. "The elucidation of invariant — and therefore predictive — principles of nature"; and
  2. " 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".

Hindcasting

The activity of hindcasting involves running a forecast model after an event has happened in order to test whether the model's simulation is valid.
In 2003, Dake Chen and his colleagues "trained" a computer using the data of the surface temperature of the oceans from the last 20 years. Then, using data that had been collected on the surface temperature of the oceans for the period 1857 to 2003, they went through a hindcasting exercise and discovered that their simulation not only accurately predicted every El Niño event for the last 148 years, it also identified the looming foreshadow of every single one of those El Niño events.

Retrodiction

The activity of retrodiction involves moving backwards 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.
Given that retrodiction is a process in which "past observations, events and data are used as evidence to infer the process the produced them" and that diagnosis "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 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 — 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 backwards 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:

In 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. 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. John Searle imagines a man in a locked room who receives written sentences in Chinese, and returns written sentences in Chinese, according to a sophisticated instruction manual. Here, the relevant question is not whether or not the man understands Chinese, but more broadly, whether a functionalist theory of mind is correct.
It is generally hoped that there is universal agreement about the intuitions that a thought experiment elicits. 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, and ask what they would do.
For example, in the veil of ignorance, John Rawls asks us to imagine a group of persons in a situation where they know nothing about themselves, and are charged with devising a social or political organization. The use of the state of nature to imagine the origins of government, as by Thomas Hobbes and John Locke, may also be considered a thought experiment. Søren Kierkegaard explored the possible ethical and religious implications of Abraham's binding of Isaac in Fear and Trembling Similarly, Friedrich Nietzsche, in On the Genealogy of Morals, speculated about the historical development of Judeo-Christian morality, with the intent of questioning its legitimacy.
An early written thought experiment was Plato's allegory of the cave. Another historic thought experiment was Avicenna's "Floating Man" thought experiment in the 11th century. He asked his readers to imagine themselves suspended in the air isolated from all in order to demonstrate human self-awareness and self-consciousness, and the substantiality of the soul.

Possibility

In many thought experiments, the scenario would be nomologically possible, or possible according to the laws of nature. John Searle's Chinese room is nomologically possible.
Some thought experiments present scenarios that are not nomologically possible. In his Twin Earth thought experiment, Hilary Putnam asks us to imagine a scenario in which there is a substance with all of the observable properties of water, but is chemically different from water. It has been argued that this thought experiment is not nomologically possible, although it may be possible in some other sense, such as metaphysical possibility. It is debatable whether the nomological impossibility of a thought experiment renders intuitions about it moot.
In some cases, the hypothetical scenario might be considered metaphysically impossible, or impossible in any sense at all. David Chalmers says that we can imagine that there are zombies, or persons who are physically identical to us in every way but who lack consciousness. This is supposed to show that physicalism is false. However, some argue that zombies are inconceivable: we can no more imagine a zombie than we can imagine that 1+1=3. Others have claimed that the conceivability of a scenario may not entail its possibility.

Interactive thought experiments in digital environments

The philosophical work of Stefano Gualeni focuses on the use of virtual worlds to materialize thought experiments and to playfully negotiate philosophical ideas. His arguments were originally presented in his 2015 book
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.
Among the most visible thought experiments designed by Stefano Gualeni:
Other examples of playful, interactive thought experiments:
Scientists tend to use thought experiments as imaginary, "proxy" experiments prior to a real, "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, such as Einstein's thought experiment of chasing a light beam, leading to special relativity. This is a unique use of a scientific thought experiment, in that it was never carried out, but led to a successful theory, proven by other empirical means.

Causal reasoning

The first characteristic pattern that thought experiments display is their orientation
in time. They are either:
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:
Thought experiments have been used in a variety of fields, including philosophy, law, physics, and mathematics. In philosophy they have been used at least since classical antiquity, some pre-dating Socrates. In law, they were well-known to Roman lawyers quoted in the Digest. In physics and other sciences, notable thought experiments date from the 19th and especially the 20th century, but examples can be found at least as early as Galileo.

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. In this paper, starting from certain philosophical assumptions, on the basis of a rigorous analysis of a certain, complicated, but in the meantime assertedly realizable model, he came to the conclusion that quantum mechanics should be described as "incomplete". Niels Bohr asserted a refutation of Einstein's analysis immediately, and his view prevailed. After some decades, it was asserted that feasible experiments could prove the error of the EPR paper. These experiments tested the Bell inequalities published in 1964 in a purely theoretical paper. The above-mentioned EPR philosophical starting assumptions were considered to be falsified by empirical fact.
Thus thought experiments belong to a theoretical discipline, usually to theoretical physics, but often to theoretical philosophy. In any case, it must be distinguished from a real experiment, which belongs naturally to the experimental discipline and has "the final decision on true or not true", at least in physics.

Examples