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Brains in a Vat -
The skeptical hypothesis that one is a brain in a vat with systematically delusory experience is modelled on the Cartesian Evil Genius hypothesis, according to which one is a victim of thoroughgoing error induced by a God-like deceiver. The skeptic argues that one does not know that the brain-in-a-vat hypothesis is false, since if the hypothesis were true, one's experience would be just as it actually is. Therefore, according to the skeptic, one does not know any propositions about the external world (propositions which would be false if the vat hypothesis were true).
Hilary Putnam provided an apparent refutation of a version of the brain-in-a-vat hypothesis, based upon semantic externalism. This is the view that the meanings and truth conditions of one's sentences, and the contents of one's intentional mental states, depend upon the character of one's external, causal environment. This entry is primarily focussed upon evaluating the Putnamian considerations that seem to show that one can know that one is not a brain in a vat. - from the Standford Encyclopedia of Philosophy, http://plato.stanford.edu/entries/brain-vat/
