A philosophical conundrum posed by Hilary Putnam, which neatly summarises mainly of the problems with the cartesian theory of knowledge (See: RichardRorty). In essence, it postulates that a brain in a jar fed all the correct sensory input would be unable to distinguish its experiences from those of any normal person (assuming no qualiative distinction between sensory input in either case). Putnam suggests the conundrum may be solved by noting that a statement 'I am a brain in a vat' is false in the event that one is not and still false in the event that one is, since the statement refers to the sensory knowledge available to the speaker, and are therefore making reference to sensory images rather than reality.
More specifically, modern science takes the view that reality exists as a logical necessity since to assert otherwise is to invalidate one's own capacity to make observations. Following KarlPopper science is as concerned with establishing results with reference to the internal consistency of formal systems as with observation.
See other InterestingMemes.