The “Brain in a Vat” Thought Experiment Described, With a Nitpick.

The Thought Experiment.

Matthew Christopher Bartsh
5 min readMay 11, 2022


The “Brain in vat” idea is that each person has no way of knowing that he (or she) is not a brain in a container in some sort of lab. It’s a bit like the (much later) idea of the movie called The Matrix and may have inspired that movie. It’s an old philosophical thought experiment.

The brain would float in a bath of liquid similar to the cerebrospinal fluid that is normally found inside the skull, and nutrient-rich, oxygenated blood would be pumped into this brain via arteries connected to some sort of life support system which would also remove the carbon dioxide and other waste from the blood when it came out of the veins of that brain.

Thus the brain would be kept alive and healthy.

Furthermore, the owner of the vat would be stimulating all the nerves leading into the brain in such a way as create the illusion of a normal everyday situation, where one can feel and see (and smell et cetera) one’s own body and the rest of the world.

Likewise, the nerve signals that the brain sends out along the nerves that come out of the brain are detected by the vat owner’s technology and used to modify the incoming nerve signals appropriately. For example, if it seems that you look up by turning your eyeballs upwards in their sockets, and thus see what is in the sky, what could really be happening, if you were in fact a brain in a vat, would be that the nerve signals coming out of your brain along the nerves that would, if you had eyes, control the eye muscles are detected and instead used to change the signals that are being sent in via the optic nerves so that you get the impression that you see the sky and whatever is in it.

It’s up to the vat owner to choose what you see in the sky. It could be bird, a plane, or anything else that would be indistinguishable from reality. Other nerve signals would be sent in from the nerves that would, if you had eyes, convey sensations from the outside of eyeballs, and the eyelids, and the eye muscles, so that it felt it exactly like your nonexisten eyeballs were turning upwards in their nonexistent sockets.

The idea is that there is nothing you can do to find out whether you are this sort of brain in vat, because all your experiences will be exactly the same as if you had a body and were controlling it.

The nitpick.

It’s fun to think about this, and a lot of people have thought about it, and written about it, and everyone seems to have failed to notice that there is in fact a way to tell. All you have to do is to attempt to directly stimulate your brain, for example by banging your head against a brick wall, in turn causing your brain to bang against the inside of your skull, causing the cells of your brain to get squashed against the inside of the skull and each other, which causes chemicals to get released into the brain that cause unconsiousness. “Knock yourself out”, in other words.

If you are a brain in a vat, you won’t go unconscious, because all that the vat owner can do is make you feel your head hitting the brick wall, which would hurt, but not cause unconsciousness. I guess the vat owner could reach into the vat and slam the brain into the side of the vat, but that wouldn’t be easy with all the artificial tubes and so on surrounding the brain in the vat and this issue has never as far as I know been considered. And the exact pattern of pressure waves that exist inside of the skull would somehow need to be recreated inside the vat, without a skull, and in spite of the all those tubes and so on surrounding the brain.

Likewise, in the Matrix movie, one can reasonably ask what happens when Keanu Reaves gets knocked out? There doesn’t seem to be any apparatus visible in the movie that could provide the needed bang on the head to produce the resulting unconsciousness experienced. Note that the same argument can be made for a bang on the head that merely causes a slight alteration of consciousness (due to chemicals released in the brain by colliding or shocked synapses), such as a reduced ability to answer questions on a “concusssion” detecting cognitive test of the type an American football player is given after he takes a hard fall where a “concussion” is suspected.

Of course, you can postulate that the vat owner has the means to bang the brain in the vat, but then what about when the concussion comes from a glancing blow, and consequent sudden rotation of the brain? You can say the vat owner can rotate the brain at will. But this is not so easy, especially with all those tubes attached to it. It’s starting to look more like a parallel universe with a God in charge of it than a vat with a brain in it.

Besides concussion, there, is drugs. When you take alcohol, LSD, or magic mushrooms, molecules go into your blood and then into your brain, causing a change in how the brain works. The vat owner can only cause you to see and hear things, not feel differently, unless you postulate that the vat owner has at his (or her or its) disposal every drug and can put it into the blood supply of the brain.

Or what if you dripped LSD into your eye? How would the vat owner make the LSD get to your brain in exact right concentration to match what would happen in real life if you did that?

Then there’s brain stimulation by a brain surgeon using electrodes. Not easy for the vat owner to reproduce that exactly without taking the brain out of the vat, which would produce other changes to the brain.

And what about magnetic stimulation. Intracranial magnetic stimulation causes the brain to behave differently. Again, difficult for the vat owner to replicate the exact magnetic fields and electrical currents without a skull. Another way it doesn’t look so much like a brain in a vat as a universe with God.

Photo of a vat by Jeff Frank on Unsplash



Matthew Christopher Bartsh

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