Thought experiment

A thought experiment is an imaginary scenario designed to test an argument or a theory. These mental exercises allow thinkers to explore ideas that are difficult, impossible, or unethical to perform in the real world. Instead of using physical tools, a person uses their mind to explore a hypothetical situation. This process often involves rearranging existing empirical data in a new way. By looking at facts from an unusual perspective, thinkers can draw new inferences. This method helps us explain, predict, and control events more effectively.

To function, a thought experiment relies on a specific way of reasoning. This is known as subjunctive reasoning, or the use of irrealis moods. This means asking questions such as, "What might happen if..." or "What would have happened if...". The goal is to elicit an intuitive or reasoned response about the nature of a concept. In philosophy, these scenarios often target ideas like morality or the nature of the mind. A successful thought experiment is one where the resulting intuitions are widely shared by others. However, philosophers often disagree on the responses these scenarios produce.

Thought experiments can serve several different theoretical purposes. They might be used to confirm a theory that is already widely accepted. They can also be used to establish an entirely new theory. Sometimes, they are used to challenge or refute a prevailing theory. One common method for this is called reductio ad absurdum. This is a proof by contradiction where a thinker shows that a theory leads to an absurd result.

By showing these absurdities, the thinker can demonstrate that the original theory must be flawed.

History shows that these mental exercises have been used since antiquity. In ancient Greece, thinkers used them to pose philosophical questions. Some of these patterns of mathematical proof existed even before Euclidean mathematics. In the 11th century, the thinker Avicenna proposed the "floating man" experiment. He asked people to imagine themselves suspended in the air without any sensations. He used this to demonstrate human self-awareness and the existence of the soul. In the 17th century, Galileo used logical demonstrations to study physics.

Many famous examples illustrate the power of these ideas. In 1638, Galileo described a way to understand falling objects. He imagined attaching a small stone to a larger one to see how they would fall. This logical demonstration helped advance the history of modern science. In 1935, Erwin Schrödinger devised a famous experiment involving a cat. He used the scenario to show how the Copenhagen Interpretation of quantum mechanics could lead to absurd conclusions. In this experiment, a cat is described as being in a superposition of being both alive and dead.

Another example is Maxwell's demon, which imagines a being that could violate the second law of thermodynamics.

In the 19th and 20th centuries, the terminology for these ideas became more defined. Hans Christian Ørsted was the first to use an equivalent German term in 1820. By 1883, Ernst Mach used the term to describe imaginary experiments. Mach suggested that these were a necessary precondition for physical experiments. He asked his students to provide explanations when their real results differed from their imaginary ones. The English term "thought experiment" was later created as a calque of the German term. It first appeared in an 1897 English translation of Mach's work.

Today, thought experiments are applied across many diverse fields. Scientists use them as "proxy" experiments to test theories before conducting physical ones. In the legal field, the term "hypothetical" is often used as a synonym. Lawyers and legal scholars use these scenarios to assess responsibility or culpability. Philosophers use them to explore ethics, such as whether an action is moral based on its consequences. They also use them to study political organization, like John Rawls's "veil of ignorance." Even science fiction writers use these structured questions to build their worlds.