10 Thought Experiments Explained Simply: What Each One Tests and Where It Comes From

Start hereImagine you had only ever seen shadows on a wall. Would you take them for the real world? Plato asked that question in the Republic, and people still use scenes like it to test ideas.

A thought experiment is an imagined situation used to test an idea: you follow what would happen, instead of running a real experiment. The Stanford Encyclopedia of Philosophy calls thought experiments “devices of the imagination”.

Below are ten famous ones in four groups. Each card says what the scene is, what the sources say about it, and how settled the question is. Five more, including the ship of Theseus and the trolley problem, already have pages of their own, linked near the end.

01What a thought experiment is

A thought experiment asks what would follow if a scene were true, and people still discuss how much thinking alone can teach.

The encyclopedia says thought experiments suggest we can learn about the real world “by virtue of merely thinking about imagined scenarios”. It then asks how we can learn about reality “(if we can at all), just by thinking”.

Ernst Mach is commonly credited with introducing the term. Its history goes back at least to the Danish word “Tankeexperiment”, as Hans-Christian Ørsted used it, and the encyclopedia quotes a scholar who found the word in use by 1811.

The historian of physics Pierre Duhem is “almost alone”, the encyclopedia says, in what has been understood as an outright condemnation of scientific thought experiments. In his view, a thought experiment is “no substitute for a real experiment”.

Our reading: a thought experiment is a careful question, not a finished answer.

For example, asking “what would I do if I won the lottery?” can show what you value, without buying a ticket.

02Doubting what you see

Three of these scenes ask whether the world you see is the whole truth, or real at all.

1. Plato’s cave

Prisoners who have only ever seen shadows on a wall would take the shadows for the real world.

In Book VII of Plato’s Republic, Socrates tells Glaucon an allegory about people living in a cave. In Benjamin Jowett’s translation, they have been there “from their childhood”, chained so that they can only see in front of them. For them, Socrates says, “the truth would be literally nothing but the shadows of the images”.

If one of them went up into the light and came back, the others would say that “up he went and down he came without his eyes”. Socrates suggests they would even put to death anyone who tried to lead them up to the light.

He then gives his own reading, offered, he says, as “my poor belief”: the prison-house is “the world of sight”, and the climb out is “the ascent of the soul into the intellectual world”. Our page on quotes from Socrates, Plato and Aristotle looks at how the cave is quoted today.

For example, if you knew a city only from photos, you might take the photos for the place until you walked its streets.

How sure: Strong the scene and Socrates’ own reading of it are in the text, in Jowett’s translation.

2. Descartes’ deceiving demon

What if a powerful deceiver were feeding you a false world, so that even your body was an illusion?

René Descartes set this out in his Meditations, first published in 1641 with six sets of objections and replies. In John Veitch’s translation, he first notes that there are “no certain marks” to tell waking from sleep.

Then he imagines “some malignant demon, who is at once exceedingly potent and deceitful”. He even resolves to think of himself as “without hands, eyes, flesh, blood, or any of the senses”.

The doubt sweeps away almost everything, but one thing is left standing. In the second Meditation he finds that “I am, I exist” must be true each time he says it or thinks it.

The encyclopedia says the demon story itself is not what matters: the point is realising that, “for all we know, our cognitive wiring is flawed”.

For example, a vivid dream can feel completely real until you wake up.

How sure: Strong the demon and the “I am, I exist” step are in the text, in Veitch’s translation.

3. The brain in a vat

Could you be a brain kept alive in a vat, with a computer feeding you every experience?

The encyclopedia says the sceptical scenarios most widely used today are versions of this: a “bodiless brain in a vat” stimulated by “a super-computer”.

In 1981 Hilary Putnam argued that the idea that we are, and always have been, brains in vats is “self-refuting”. His reason turns on what our words can refer to. On his view, if we had always been brains in vats, our sentence “we are brains in a vat” would mean something like “brains in a vat in the image”.

Thomas Nagel replied that, even if the argument works, it raises a new worry: a brain in a vat still could not truly think that it is one. The encyclopedia says Putnam’s argument started “a stream of illuminating literature” on how to rebuild and judge it.

For example, a very detailed video game can feel like a real place while you play it.

How sure: Unsettled Putnam’s argument started a stream of work on how to rebuild and judge it, and replies like Nagel’s are still part of that discussion.

03What a mind is

Three scenes test what a mind is: whether knowing every fact is enough, whether following rules is understanding, and whether the senses share what they know.

4. Mary’s room

A scientist who knows every physical fact about colour, but has only ever seen black and white, sees colour for the first time: does she learn something new?

Frank Jackson set out the case in 1982. In Jackson’s own description, which the encyclopedia quotes, Mary has had to study the world “from a black and white room via a black and white television monitor”. She has “all the physical information there is to obtain” about what happens when we see ripe tomatoes.

Jackson states the intuition this way: “It seems just obvious that she will learn something about the world and our visual experience of it”. If she does, the argument goes, then not all information is physical information. That would be a problem for physicalism, the view that everything is physical. The encyclopedia calls Mary’s room “one of the most discussed arguments against physicalism”.

Jackson himself later changed his mind, and in 1995 argued that something must be wrong with the argument. One reply says Mary gains no new fact, only “a bundle of abilities”, such as being able to imagine, remember and recognise colours.

For example, reading every review of a dish is not the same as tasting it.

How sure: Unsettled it is one of the most discussed arguments against physicalism, and even Jackson changed his mind about it.

5. The Chinese room

Someone following a rule book to answer messages in Chinese, without knowing any Chinese, could look fluent from outside.

The American philosopher John Searle first published the argument in a 1980 article. He imagines himself alone in a room, following a computer program for responding to Chinese characters slipped under the door. People outside would wrongly suppose there is a Chinese speaker in the room.

His narrow conclusion is that programming a computer may make it appear to understand language but could not produce real understanding. One reply, called the Systems Reply, says that while the man does not understand Chinese, the system as a whole does.

Since AI systems based on large language models drew wide attention in late 2022, the encyclopedia asks whether they make any difference to the argument. Our page on how large language models work explains what those systems do, in plain words.

For example, a traveller reading answers from a phrasebook can reply correctly without understanding the words.

How sure: Unsettled the encyclopedia says there is still no consensus on whether the argument is sound.

6. Molyneux’s problem

If a person born blind could suddenly see, could they tell a cube from a sphere by sight alone?

William Molyneux sent the question to John Locke in a letter on 7 July 1688. For reasons unknown, Locke never replied to that letter. Molyneux asked again in a letter dated 2 March 1693, and Locke quoted it almost word for word in the second edition of his Essay, in 1694. Molyneux’s wife had lost her sight in the first year of their marriage.

Molyneux answered no, and Locke agreed: the blind man “at first sight, would not be able with certainty to say which was the globe, which the cube”.

Surgeons had reported on newly sighted patients since 1728, but those early reports were anecdotal and hard to compare. In 2011, a study of people born blind whose sight was restored found “a lack of immediate transfer”, but the link between touch and sight “developed rapidly”. The encyclopedia describes the study: five people born blind had their vision restored by cataract surgery. All five failed the touch-to-vision test at first, barely better than guessing. Three of them succeeded when tested after a few days of “natural real-world experience”. The encyclopedia adds that critics often question how robust the new sight of these patients was.

Even so, the encyclopedia calls it an open question whether Molyneux’s original question would also be answered “no”.

For example, you might know a tune by ear and still not recognise it written down as notes.

How sure: Moderate the study tested five people, and the encyclopedia says the original question is still open.

04How to live

Two scenes ask what we value: being good when no one can see, and real life over pleasant feelings.

7. The ring of Gyges

If a ring could make you invisible, would you still act fairly?

In Book II of Plato’s Republic, Glaucon tells of Gyges, “a shepherd in the service of the king of Lydia”. When he turned the ring, “instantly he became invisible to the rest of the company”. He then “seduced the queen, and with her help conspired against the king and slew him, and took the kingdom.”

Glaucon argues that no one would be of “such an iron nature” as to stay just with such a ring. He sums up the view: “wherever any one thinks that he can safely be unjust, there he is unjust.”

But Glaucon is putting the common view, not his own. He says so plainly: “since I myself am not of their opinion”. What he wants is to hear justice “praised in respect of itself”.

For example, would you tidy a shared kitchen if no one could ever know who did it?

How sure: Strong the story and Glaucon’s challenge are in the text, in Jowett’s translation.

8. The experience machine

Would you plug into a machine that gave you any experiences you wanted, for the rest of your life?

Robert Nozick set it out in Anarchy, State, and Utopia, published in 1974. The encyclopedia puts the scene this way: “Imagine I have a machine that I could plug you into for the rest of your life.” You “would not know you were on the machine”.

The encyclopedia says Nozick thinks it would be a big mistake to plug in. His reason, in his words, is that “something matters to us in addition to experience”. We want to do certain things, he says, not just have the experience of doing them.

The scene is aimed at hedonism, the view that only pleasure has worth or value. The encyclopedia calls it a weighty objection to hedonism, and to the view that well-being consists only in conscious states.

It also says that quickly dismissing hedonism on the strength of the machine “is not methodologically sound”. One reply open to hedonists is to accept that the objection has force, but to say it rests on “common sense” intuitions.

For example, watching a film of a mountain climb is not the same as climbing it, however good the film.

How sure: Unsettled the encyclopedia calls it a weighty objection, but warns against dismissing hedonism quickly on its strength.

05Testing nature in your head

Scientists use thought experiments too, and two famous ones changed how people thought about falling and gravity.

9. Galileo’s falling stones

If heavy things fell faster, a heavy stone tied to a light one would have to fall both faster and slower.

Galileo set out the argument in Two New Sciences, published in Holland in 1638. In Henry Crew and Alfonso de Salvio’s 1914 translation, the character Salviati says the faster body “will be partly retarded by the slower”.

Tied together, the pair is heavier than the heavy stone alone, so it should fall faster. Yet the light stone should also hold it back, so it should fall slower too. The encyclopedia sums it up: the joined body “would have to fall both faster and slower” than the heavy one alone.

Salviati concludes that “even without further experiment” a short argument shows that a heavier body does not fall faster than a lighter one of the same material. In the same scene, another speaker, Sagredo, says he has “made the test” with a cannon ball and a musket ball. The encyclopedia notes that Galileo’s texts are “full of illustrative scenarios he does not pretend to have carried out”.

For example, tie a light ball to a heavy one in your mind and ask which way the light one should pull.

How sure: Strong the argument is in Galileo’s own text, in the 1914 English translation.

10. Einstein’s chest

A person inside a closed box pulled upward with a constant force, so that it keeps speeding up, would feel just as if gravity were holding them down.

Albert Einstein described it in Relativity, first published in German in 1917; we read Robert W. Lawson’s authorised translation. He asks us to “imagine a spacious chest resembling a room with an observer inside who is equipped with apparatus.”

Something then pulls the chest with a constant force, so that it moves “upwards” with a uniformly accelerated motion. The man inside concludes that he is in a gravitational field that does not change over time. Einstein asks: “Ought we to smile at the man and say that he errs in his conclusion?” He answers that we should not, if we wish to remain consistent.

So from inside the box, being pulled and being held by gravity feel the same. A footnote in this English edition says the bending of light the theory predicted was “first confirmed” during the solar eclipse of 29 May 1919. The encyclopedia calls the scene “Einstein’s elevator”, listing it with Galileo’s falling bodies and Newton’s bucket.

For example, in a lift that sets off upward, you feel heavier for a moment.

How sure: Strong the scene is in Einstein’s own book, in Lawson’s authorised translation.

06Already on our site

Five more thought experiments have their own pages on our site, so here they get one line each.

• The ship of Theseus. Plutarch describes an old debate about a ship whose timbers were replaced, and the second ship, built from the old planks, is Thomas Hobbes’s addition. See our page on mental models from philosophy.
• The trolley problem. Philippa Foot wrote the case it descends from in 1967, and in her version it was a runaway tram. See our page on where the trolley problem comes from.
• Buridan’s ass. A donkey is stuck between two equal piles of hay, but, in the encyclopedia’s words, the example is “nowhere to be found in Buridan’s writings”. See mental models from philosophy.
• The veil of ignorance. In John Rawls’s version, the principles of justice are chosen behind a “veil of ignorance”, and John Harsanyi discussed the problem before Rawls. See mental models from philosophy.
• The dragon in the garage. Chapter 10 of Carl Sagan’s The Demon-Haunted World opens with a thought experiment: “A fire-breathing dragon lives in my garage.” See our summary of the book.

07How to use one yourself

Our suggestion: you can run a small thought experiment on any idea you feel sure of.

1. Picture one clear case, with as few details as you can.
2. Change one detail at a time, and ask what would follow.
3. Check the result against the real world when you can, the way researchers in 2011 tested Molyneux’s question with people whose sight had been restored.

For example, if you are sure a new rule at work is fair, imagine you were the person it affects most, then change who that person is.

08Where to learn more

• Plato’s cave and how it is quoted: our page on quotes from Socrates, Plato and Aristotle.
• The ship of Theseus, Buridan’s ass and the veil of ignorance: our page on mental models from philosophy.
• The trolley problem: our page on where the trolley problem comes from.
• What chatbots do, for the Chinese room: our page on how large language models work.
• The prisoner’s dilemma, another imagined scene: our page on game theory in plain words.
• For depth on any card, the Stanford Encyclopedia of Philosophy entries listed below are free to read and go much further.

09Quick answers

10Use this

11Quick quiz

How we checked this · Last checked November 2026

• We read the Stanford Encyclopedia of Philosophy entry on each thought experiment and, where the scene comes from an old book, the book itself in a named translation (Jowett for Plato, Veitch for Descartes, Crew and de Salvio for Galileo, Lawson for Einstein).
• Nozick's book is not freely readable, so the experience machine is given in the encyclopedia's words, and his own phrases are the ones the encyclopedia quotes.
• The ship of Theseus, the trolley problem, Buridan's ass, the veil of ignorance and the dragon in the garage already have pages on our site, so they get one line each here.

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