Thought experiments

Imagined cases that test a concept by pushing it until something gives: 13 of the most famous, each with the case in a sentence, what it is meant to show, the main replies, and the original text where the library has it. Some you can run in the lab.

Plato · c. 375 BCEThe allegory of the cavePrisoners chained since childhood in an underground den face a wall on which they see only the shadows of objects carried behind them past a fire. One is freed and forced up into the sunlight, and slowly learns to see the real things; if he went back down, his old companions would think the climb had ruined him, and might kill him.
Edmund Gettier · 1963The Gettier casesSmith has strong evidence that Jones will get the job and that Jones has ten coins in his pocket, so he concludes that the man who will get the job has ten coins in his pocket. In fact Smith gets the job, and, without knowing it, he has ten coins in his own pocket. His belief is true and justified, yet it seems he does not know it.
Nāgasena, as recorded in the Milindapañha (author unknown) · c. 100 BCE–200 CENāgasena's chariotKing Milinda asks the monk Nāgasena his name, and Nāgasena says it is only a designation, since no permanent self is involved. He then asks how the king came, and when Milinda says by chariot, asks whether the chariot is its pole, its axle, its wheels, all of them together, or something apart. Milinda says it is none of these, yet he did come by chariot, and Nāgasena says a person is the same.
Plutarch · c. 75 CEThe Ship of TheseusThe ship in which Theseus sailed home is kept in Athens as a memorial. As its planks rot they are replaced one by one, until no original plank remains. Is it still the ship of Theseus? And if someone gathers the discarded planks and rebuilds them into a ship, which of the two is the original?
Alan Turing · 1950The Turing testAn interrogator holds text conversations with two hidden partners, one a human and one a machine, and must decide which is which. If the interrogator cannot reliably tell them apart, Turing proposed, the machine has done well enough at the question “can machines think?” that the question can be replaced by this one.
Frank Jackson · 1982Mary's roomMary is a brilliant scientist who has spent her whole life in a black-and-white room, learning everything physical there is to know about colour vision, from the physics of light to the neuroscience of the eye and brain. One day she leaves the room and sees a red tomato for the first time.
David Chalmers · 1996The philosophical zombieImagine a being physically identical to you, atom for atom, which behaves exactly as you do and says everything you say, including “I am conscious”, but which has no inner experience at all. There is nothing it is like to be it.
Hilary Putnam · 1981The brain in a vatA scientist removes your brain, keeps it alive in a vat of nutrients, and connects its nerve endings to a computer that feeds it exactly the signals your senses would have sent. Everything seems normal to you. You see your room, feel your hands, read this sentence.
Philippa Foot · 1967The trolley problemA runaway trolley will kill five workmen unless you pull a lever that sends it onto a side track, where it will kill one. Most people say you may pull the lever. But if the only way to stop the trolley is to push a stranger off a footbridge into its path, most say you may not, though the arithmetic is the same.
Robert Nozick · 1974The experience machineNeuroscientists can plug you into a machine that gives you any experiences you want, indistinguishable from real life. You could feel that you are writing a great novel, falling in love or climbing Everest, while in fact you float in a tank with electrodes in your brain. You would not know you were plugged in.
John Rawls · 1971The veil of ignoranceYou are to choose the basic rules of a society, but behind a veil that hides who you will be in it. You do not know your class, sex, race, talents, health or even your idea of a good life. Whatever principles you pick, you must then live under them as whoever you turn out to be.
Nick Bostrom · 2003The simulation argumentSuppose a civilisation could one day run vast numbers of detailed simulations of minds like ours. Then simulated minds would vastly outnumber biological ones, and nothing in your experience would tell you which kind you are.
John Searle · 1980The Chinese roomA man who knows no Chinese sits in a closed room with a rule book, written in English, for matching incoming Chinese characters with outgoing ones. Following the rules, he sends out replies that convince native speakers outside that someone inside understands Chinese. He understands nothing.