Hilary Putnam Argued You Cannot Coherently Say You Are a Brain in a Vat

Descartes asked how you would know if everything you experienced were the work of an evil demon. The modern version is a brain floating in a vat of nutrients, fed inputs by a computer, having all the experiences your brain is having now. The challenge is that you cannot, from the inside, tell the two scenarios apart.

In 1981, in Reason, Truth and History, Hilary Putnam tried to dissolve the puzzle. His move was about reference. What does the word "brain" refer to? Putnam argued that words pick out the things they pick out because of a causal chain reaching from speaker to object. "Tree" refers to trees because the people who first used the word were standing near actual trees. The chain has to run through the world.

Now take the brain-in-a-vat scenario seriously. The brain has always been in the vat. It has never been in causal contact with a real tree, or a real brain, or a real vat — only simulated images of them. By Putnam's account, the words it thinks cannot refer to those things. When the vat-brain thinks "I am a brain in a vat," the words pick out at most the simulated brain in the simulated vat — which is not what the brain in fact is. The sentence comes out false. There is no reading of it on which it is true.

The argument has been picked apart for forty years. Critics object that it presupposes the very theory of meaning it needs. But it remains the cleanest reply to global skepticism on offer: we cannot rule out being deceived, but we can show the deception cannot coherently announce itself.