The Brain in a Vat

Before reaching his famous certainty in What the Doubt Cannot Throw Out, Descartes asked how he knew he was not dreaming, since dreams can feel completely real. Then he went further: suppose a powerful evil demon were deceiving him about everything outside his mind.

The modern version is the brain in a vat. A scientist keeps your brain alive in a vat and feeds it signals exactly like those from a real body and world. Your experience would be just as it is now. How could you tell the difference? If you cannot, the skeptic argues, you cannot know you have hands.

G. E. Moore replied that he knew he had hands more surely than he knew any premise of the skeptic’s argument. Hilary Putnam argued that a brain in a vat could not even think the thought, since its words would not refer to real brains or vats.

The perfect game

Sam plays a virtual reality game so convincing that, after hours inside, he briefly forgets the headset. When he takes it off, the room seems oddly flat.

He wonders how he knows this room is not another layer of the game. He cannot prove it. But he notices that the question changes nothing about whether to eat dinner or call his sister. Like Moore, he trusts the hands in front of him more than the doubt.

Try this today Pick one thing you are sure of. Ask what evidence you have for it that a perfect simulation could not also give you.