My Favorite Thought Experiments

Hans Christian Ørsted coined the German term Gedankenexperiment (transl. thought experiment) in 1812

The Trolly Problem

Imagine you are standing beside some tram tracks. In the distance, you spot a runaway trolley hurtling down the tracks towards five workers who cannot hear it coming. Even if they do spot it, they won’t be able to move out of the way in time. As this disaster looms, you glance down and see a lever connected to the tracks. You realize that if you pull the lever, the tram will be diverted down the second set of tracks away from the five unsuspecting workers. However, down this side track is one lone worker, just as oblivious as his colleagues. So, would you pull the lever, leading to one death but saving five?

Achilles and the tortoise

Two-and-a-half millennia ago, the Greek philosopher Zeno of Elea apparently proved that motion is an illusion. One of his paradoxes sets fleet-of-foot Achilles to chase a tortoise that has a small head start. Achilles can never catch the tortoise, argued Zeno, because first he must reach the point where the tortoise started, but by then the tortoise has moved on to a new position. So then Achilles must run there — by which time the tortoise has moved on again. Is it true that Achilles will never catch the tortoise? If not, how so?

Mary the Colorblind Neuroscientist

Mary is a brilliant scientist who is, for whatever reason, forced to investigate the world from a black and white room via a black and white television monitor. She specializes in the neurophysiology of vision and acquires, let us suppose, all the physical information there is to obtain about what goes on when we see ripe tomatoes, or the sky, and use terms like ‘red’, ‘blue’, and so on. She discovers, for example, just which wavelength combinations from the sky stimulate the retina, and exactly how this produces via the central nervous system the contraction of the vocal cords and expulsion of air from the lungs that results in the uttering of the sentence ‘The sky is blue’…What will happen when Mary is released from her black and white room or is given a color television monitor? Will she learn anything or not?

The Experience Machine

Suppose there was an experience machine that would give you any experience you desired. Superduper neuropsychologists could stimulate your brain so that you would think and feel you were writing a great novel, making a friend, or reading an interesting book. All the time you would be floating in a tank, with electrodes attached to your brain. Should you plug into this machine for life, preprogramming your life experiences?… Of course, while in the tank you won’t know that you’re there; you’ll think that it’s all actually happening…Would you plug yourself in?”


Suppose a man is out for a walk one day when a bolt of lightning disintegrates him. Simultaneously, a bolt of lightning strikes a marsh and causes a bunch of molecules to spontaneously rearrange into the same pattern that constituted that man a few moments ago. This “Swampman” has an exact copy of the brain, memories, patterns of behavior as he did. It goes about its day, works, interacts with the man’s friends, and is otherwise indistinguishable from him. So, is the Swampman the same person as the disintegrated fellow?

Thompson’s violinist

“You wake up in the morning and find yourself back to back in bed with an unconscious violinist. A famous unconscious violinist. He has been found to have a fatal kidney ailment, and the Society of Music Lovers has canvassed all the available medical records and found that you alone have the right blood type to help. They have therefore kidnapped you, and last night the violinist’s circulatory system was plugged into yours, so that your kidneys can be used to extract poisons from his blood as well as your own. If he is unplugged from you now, he will die; but in nine months he will have recovered from his ailment, and can safely be unplugged from you”. Thus, are you obligated to keep the musician alive, or do you cut him loose and let him die because you want to?

The ticking time bomb

You are a senior law officer and a terrorist group has announced that it has hidden a nuclear bomb in your building. It will explode in half an hour unless you stop it. You have caught the terrorist group’s leader, who knows where the bomb is but will not tell you. Torturing him is guaranteed to provide you with the information you need. Do you torture him to save the lives of everyone else in your building?

The problem of Theseus’ ship

A ship’s sail breaks, so it is replaced. A year later, its floorboards start to rot, so they are replaced too. Eventually, each of the ship’s components is replaced and none of its original parts remain. Is it the same ship? Alternatively, the same thing happens to the cells in your body; every seven years every part of you is replaced. Are you the same person you were seven years ago?




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Arunabh Sarkar

Arunabh Sarkar

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