Why we aren't brains in vats

Putnam argued that words derive their meanings from cause-and-effect relations to the world around us. And our use of words relies on the assumption that normal causal relations exist between out mental states and objects. If we are really vat-brains and such causal relations do not obtain, then in the sentence "we are brains in vats" the words don't really mean anything. Hence the sentence "we are brains in vats" can never be true.