How do you know you're not just a brain in a jar being fed a fake world?

Imagine your brain removed from your body, suspended in nutrient fluid, and wired to a supercomputer feeding it perfectly convincing fake sensations — you'd have no way to tell from the inside. Hilary Putnam updated Descartes's evil demon with this in 1981, arguing a brain in a vat couldn't even meaningfully think 'I am a brain in a vat,' since its words would refer only to the simulation. The thought experiment is often cited as the philosophical seed of The Matrix.

— Hilary Putnam, Reason, Truth and History — Cambridge University Press, 1981
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