As the contemporary philosopher Martin Cohen puts it, "much of modern physics is built not upon measurement but on thought experimentation". As Cohen argues, the Renaissance period and the Enlightenment were characterized by breakthroughs in ways of seeing the world, not merely by new methods (and tools) for 'measuring' it.
Thus it is that perhaps the key experiment in the history of modern science, again toppling the lofty but inaccurate view of Aristotle, 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 clearly a logical demonstration, using the 'thought experiment' technique. The 'experiment' is described by Galileo in Discorsi e dimostrazioni matematiche (1628) (literally, 'Mathematical Discourses and Demonstrations') thus:
Salviati. If then we take two bodies whose natural speeds are different, it is clear that on uniting the two, the more rapid one will be partly retarded by the slower, and the slower will be somewhat hastened by the swifter. Do you not agree with me in this opinion?
Simplicio. You are unquestionably right.
Salviati. But if this is true, and if a large stone moves with a speed of, say, eight while a smaller moves with a speed of four, then when they are united, the system will move with a speed less than eight; but the two stones when tied together make a stone larger than that which before moved with a speed of eight. Hence the heavier body moves with less speed than the lighter; an effect which is contrary to your supposition. Thus you see how, from your assumption that the heavier body moves more rapidly than ' the lighter one, I infer that the heavier body moves more slowly. Galileo on Aristotle and Acceleration. Retrieved on 2008-05-24..
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 actually are.
There are many different kinds of thought experiments. All thought experiments, however, employ a methodology that is a priori, rather than empirical, in that they do not proceed by observation or physical experiment.
If a Gedankenexperiment is going to be utilized, it is imperative to formulate all data into a truth table, where you ask yourself a question and if the answer is yes, than, you can ask yourself another question, and so on. Though if the question you pose to yourself is "no", then one must go back to field a "yes" answer. All Gedankenexperiments revolve around truth tables, and a Gedankenexperiment is an introspective analysis of the truth you're trying to answer; a Gedankenexperiment is not meant to be shared with anybody.
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.
Witt-Hansen established that Hans Christian Ørsted was the first to use the Latin-German mixed term Gedankenexperiment (lit. experiment conducted in the thoughts) circa 1812. Ørsted was also the first to use its entirely German equivalent, Gedankenversuch, in 1820.
Much later, Ernst Mach used the term Gedankenexperiment in a different way to exclusively denote the imaginary conduct of a real experiment that would be subsequently performed as a real physical experiment by his students (thus the contrast between physical and mental experimentation) with Mach asking his students to provide him with explanations whenever it happened that the results from their subsequent, real, physical experiment had differed from those of their prior, imaginary experiment.
The English term thought experiment was coined (as a calque) from Mach's Gedankenexperiment, and it first appeared in the 1897 English translation of one of Mach’s papers. Prior to its emergence, the activity of posing hypothetical questions that employed subjunctive reasoning had existed for a very long time (for both scientists and philosophers). However, people had no way of categorizing it or speaking about it. This helps to explain the extremely wide and diverse range of the application of the term "thought experiment" once it had been introduced into English.
In its broadest usage, thought experimentation is the process of employing imaginary situations to help us understand the way things really are (or, in the case of Herman Kahn’s "scenarios", understand something about something in the future). The understanding comes through reflection upon this imaginary situation. Thought experimentation is an a priori, rather than an empirical process, in that the experiments are conducted within the imagination (i.e., Brown’s (1993) "laboratory of the mind"), and never in fact.
Thought experiments, which are well-structured, well-defined hypothetical questions that employ subjunctive reasoning (irrealis moods) -- "What might happen (or, what might have happened) if . . . " -- have been used to pose questions in philosophy at least since Greek antiquity, some pre-dating Socrates (see Rescher). In physics and other sciences many famous thought experiments date from the 19th and especially the 20th Century, but examples can be found at least as early as Galileo.
Thought experiments have been used in philosophy, physics, and other fields (such as cognitive psychology, history, political science, economics, social psychology, law, organizational studies, marketing, and epidemiology). 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.
In terms of their theoretical consequences, thought experiments generally:
Thought experiments often introduce interesting, important and valuable new perspectives on old mysteries and old questions; yet, although they may make old questions irrelevant, they may also create new questions that are not easy to answer.
In terms of their practical application, thought experiments are generally created in order to:
Scientists tend to use thought experiments in the form of imaginary, "proxy" experiments which they conduct prior to a real, "physical" experiment (Ernst Mach always argued that these gedankenexperiments were "a necessary precondition for physical experiment"). In these cases, the result of the "proxy" experiment will often be so clear that there will be no need to conduct a physical experiment at all.
Scientists also use thought experiments when particular physical experiments are impossible to conduct (Carl Gustav Hempel labeled these sorts of experiment "theoretical experiments-in-imagination"), such as Einstein's thought experiment of chasing a light beam, leading to Special Relativity. This is a unique use of a scientific thought experiment, in that it was never carried out, but led to a successful theory, proven by other empirical means.
Generally speaking, there are seven types of thought experiments in which one reasons from causes to effects, or effects to causes:
Counterfactual (?contrary to established fact?) thought experiments speculate on the possible outcomes of a different past; and ask "What might have happened if A had happened instead of B?" (e.g., "If Isaac Newton and Gottfried Leibniz had cooperated with each other, what would mathematics look like today?").
Semifactual thought experiments 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.
The activities of prediction, forecasting and nowcasting attempt to project the circumstances of the present into the future (the only difference between these identically patterned activities being the distance of their speculated future from the present).
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.
The activity of backcasting involves establishing the description of a very definite and very specific future situation. It then involves an imaginary moving backwards in time, step-by-step, in as many stages as are considered necessary, from the future to the present, in order to reveal the mechanism through which that particular specified future could be attained from the present.
It is important to recognize that a major difficulty with all types of thought experiment, and particularly with counterfactual thought experiments, is that there are no formally accepted criteria for accurately measuring the risk of either Type I errors (False positive) or Type II errors (False negative) in the choice of a potential causative factor.
Whereas thought experiments in physics are intended to give us a priori knowledge of the natural world, philosophy attempts to produce a priori knowledge of our concepts:
In philosophy, a thought experiment typically presents an imagined scenario with the intention of eliciting an intuitive response about the way things are in the thought experiment. (Philosophers might also supplement their thought experiments with theoretical reasoning designed to support the desired intuitive response.) The scenario will typically be designed to target a particular philosophical notion, such as morality, or the nature of the mind or linguistic reference. The intuitive response to the imagined scenario is supposed to tell us about the nature of that notion in any scenario, real or imagined.
For example, a thought experiment might present a situation in which an agent intentionally kills an innocent for the benefit of others. Here, the relevant question is whether the action is moral or not, but more broadly whether a moral theory is correct that says morality is determined solely by an action?s consequences (See Consequentialism). John Searle imagines a man in a locked room who receives written sentences in Chinese, and returns written sentences in Chinese, according to a sophisticated instruction manual. Here, the relevant question is not whether or not the man understands Chinese, but more broadly, whether a functionalist theory of mind is correct.
It is generally hoped that there is universal agreement about the intuitions that a thought experiment elicits. (Hence, in assessing their own thought experiments, philosophers may appeal to "what we should say," or some such locution.) A successful thought experiment will be one in which intuitions about it are widely shared. But often, philosophers differ in their intuitions about the scenario.
Other philosophical uses of imagined scenarios arguably are thought experiments also. In one use of scenarios, philosophers might imagine persons in a particular situation (maybe ourselves), and ask what they would do.
For example, 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 (See the veil of ignorance). 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. Similarly, Nietzsche, in On the Genealogy of Morals, speculated about the historical development of Judeo-Christian morality, with the intent of questioning its legitimacy.
One of the earliest known thought experiments was Avicenna's "Floating Man" thought experiment in the 11th century. He asked his readers to imagine themselves suspended in the air isolated from all sensations in order to demonstrate human self-awareness and self-consciousness, and the substantiality of the soul.
Some thought experiments present scenarios that are not nomologically possible. In his Twin Earth thought experiment, Hilary Putnam asks us to imagine a scenario in which there is a substance with all of the observable properties of water (e.g., taste, color, boiling point), but which is chemically different from water. It has been argued that this thought experiment is not nomologically possible, although it may be possible in some other sense, such as metaphysical possibility. It is debatable whether the nomological impossibility of a thought experiment 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.