SOCRATES. How can you tell whether at this moment we are sleeping and all our thoughts are a dream; or whether we are awake and talking to one another in the waking state?With the advent of modern science, such questions began to garner more attention and the issue is now acknowledged as a classic epistemological difficulty.
THEAETETUS. Indeed, Socrates, I do not know how to prove the one any more than the other for in both cases the facts precisely correspond. There is no difficulty in supposing that during all this discussion we have been talking to one another in a dream; the resemblance of the two states is quite astonishing.
If we are brains in vats, then the sentence ‘We are brains in vats’ says something false (if it says anything). In short, if we are brains in vats, then ‘We are brains in vats’ is false. So it is (necessarily) false.So we can be sure that we are not brains in vats by considerations drawn from the theory of reference! This provocative diagnosis has generated a large body of philosophical literature whose ramifications are still unfolding.