A thought experiment is an imaginary situation used to explain or test an idea or theory. It is often an experiment that would be difficult, impossible, or unethical to do in real life. It can also be a made-up situation used to test how we feel about right and wrong or other important questions in philosophy.
History
The ancient Greek word δείκνυμι, deiknymi, meaning "thought experiment," was the earliest known method of mathematical proof. This approach focused on ideas rather than physical experiments and was used before the time of Euclidean mathematics.
In about 1812, Johann Witt-Hansen found that Hans Christian Ørsted was the first person to use the German term "Gedankenexperiment" to describe a thought experiment. Ørsted also introduced the term "Gedankenversuch" in 1820.
By 1883, Ernst Mach used "Gedankenexperiment" in a new way to describe imagining a real experiment that would later be tested physically by his students. Mach compared the results of real experiments with the imagined ones to help students understand scientific ideas.
The English word "thought experiment" was created as a direct translation of "Gedankenexperiment." It first appeared in an 1897 English version of one of Mach's writings. Before this, scientists and philosophers had long used hypothetical questions with imaginary reasoning, which could be described using special grammar forms called "irrealis moods." This helped explain the wide use of the term "thought experiment" once it was introduced in English.
Galileo showed that objects fall at the same speed no matter their mass. This is often believed to have been a physical test from the Leaning Tower of Pisa, but it was actually a logical demonstration using a thought experiment. Galileo described this in his 1638 work Two New Sciences.
Uses
Thought experiments are used to examine ideas and test how theories might work. They are also used in schools and for fun.
Examples include Schrödinger's cat, which was created to challenge the Copenhagen Interpretation of quantum mechanics by showing how its rules might lead to a situation where a cat is both alive and dead at the same time. Another example is Maxwell's demon, which tries to show how a tiny being might break the second law of thermodynamics.
Thought experiments often appear in science-fiction stories.
These experiments are structured questions that imagine what might happen if certain conditions change. They have been used in philosophy since ancient Greece, even before Socrates. In science, many thought experiments were developed in the 19th and 20th centuries, but some date back to Galileo.
In thought experiments, people learn new things by looking at facts in new ways or from unusual angles. For example, in Galileo’s experiment, he imagined combining objects of different weights to test how they fall.
Thought experiments are used in many areas, including philosophy, physics, and other subjects like psychology, history, and law. In law, these experiments are sometimes called "hypotheticals."
No matter their purpose, thought experiments help people understand, predict, and manage events more clearly.
In terms of their effects, thought experiments can:
Thought experiments can help people see new ideas about theories they did not know or did not agree with. However, they might also make those theories less important or create new problems that are hard to solve.
In terms of their use, thought experiments are usually created to:
Fields
Thought experiments have been used in many areas, including philosophy, law, physics, and mathematics. In philosophy, they have been used since ancient times, even before Socrates. In law, Roman lawyers mentioned in the Digest were familiar with them. In physics and other sciences, important thought experiments began in the 19th and especially the 20th century, though examples exist as early as Galileo’s time.
In philosophy, a thought experiment usually describes a made-up situation to help people think about how things might be in that situation. Philosophers may also use reasoning to support the ideas that come from the situation. These scenarios often focus on specific ideas, such as what is right or wrong, or how the mind works. The responses to these situations are meant to help people understand these ideas in real or imagined situations.
For example, a thought experiment might describe a person who intentionally kills an innocent person to help others. The question here is not whether the action is moral, but whether a theory that says morality depends only on the results of an action is correct. Another example is John Searle’s idea of a man in a locked room who receives and replies to Chinese sentences using a detailed manual. The question here is not whether the man understands Chinese, but whether a theory that says the mind works like a machine is correct.
It is usually hoped that people will agree on the ideas a thought experiment brings up. Philosophers may refer to what people "should say" when discussing their experiments. A successful thought experiment is one where most people share the same ideas. However, people often disagree about what a situation means.
Other uses of imagined situations in philosophy may also be thought experiments. For example, philosophers might imagine people in a situation and ask what they would do.
In one example, John Rawls asks people to imagine a group that knows nothing about themselves and must create a social or political system. Another example is imagining a situation called the "state of nature," used by Thomas Hobbes and John Locke to think about how governments began. Søren Kierkegaard explored the moral and religious questions in the story of Abraham and Isaac in Fear and Trembling. Similarly, Friedrich Nietzsche in On the Genealogy of Morals imagined how Judeo-Christian morality developed historically to question its validity.
An early written thought experiment was Plato’s Allegory of the Cave. Another was Avicenna’s "floating man" experiment in the 11th century. He asked readers to imagine being suspended in the air without any senses to show that people can be aware of themselves and that the soul exists.
Scientists often use thought experiments as pretend experiments before real ones (Ernst Mach believed these "thought experiments" were needed before physical experiments). In these cases, the results of the pretend experiment are often clear enough that no real experiment is needed.
Scientists also use thought experiments when real experiments are impossible (Carl Gustav Hempel called these "theoretical experiments-in-imagination"). For example, Einstein imagined chasing a light beam, which led to the theory of special relativity. This was a unique case because the experiment was never done, but the theory was later proven through other tests.
Properties
Thought experiments can be grouped based on certain features. Some are nomologically possible, meaning they follow the laws of nature. For example, John Searle’s Chinese Room scenario is considered nomologically possible.
Other thought experiments describe situations that are not nomologically possible. Hilary Putnam’s Twin Earth example asks people to imagine a substance that looks, tastes, and behaves like water but has a different chemical makeup. This scenario is not nomologically possible, though it might be considered possible in other ways, such as metaphysical possibility. Whether such impossibility affects the usefulness of the experiment is a topic of debate.
Some thought experiments involve scenarios that are metaphysically impossible, meaning they cannot exist in any way. David Chalmers suggests imagining zombies—beings identical to humans physically but without consciousness. This is meant to challenge the idea that physical processes explain all aspects of the mind. However, others argue that such scenarios are not truly imaginable, like trying to picture 1+1=3. Some also say that being able to imagine a scenario does not always mean it is possible.
A key feature of thought experiments is how they relate to time. They can be:
Another key feature is how they move through time compared to the present moment of the person imagining them. This includes:
The connection between thought experiments and real experiments can be complex. For example, Albert Einstein and his colleagues proposed the EPR paradox in 1935. They used a theoretical model to argue that quantum mechanics was incomplete. Niels Bohr disagreed, and his view was accepted. Later experiments, like those testing Bell inequalities, showed that the EPR assumptions were incorrect. These experiments, such as those by Alain Aspect, proved that the EPR model was not valid.
Thought experiments are often part of theoretical disciplines, such as theoretical physics or theoretical philosophy. Unlike real experiments, which are tested in the physical world and provide final answers, thought experiments are purely theoretical.
Some thought experiments are interactive, allowing people to explore different choices and outcomes within a story or through a computer program.
The Internet has created new ways to use thought experiments. Stefano Gualeni, in his 2015 book Virtual Worlds as Philosophical Tools, argues that virtual worlds, like those in video games, can help explore philosophical ideas. He believes digital media can expand the way philosophy is studied, especially when testing ideas that require imagining non-human or non-actual experiences.