Abstract

The argument that has inspired much of the recent discussion of the logical relationship between these views is found in Putnam : If externalism is true, then if S were a brain in a vat, S’s utterances of the sentence “I am a brain in a vat” would not express the proposition that S is a brain in a vat. S’s use of the words “brain” and “vat” would not refer to a real brain or vat, just as, in a Twin Earth-style thought experiment, the Twin Earthling cannot refer to water with his use of the term “water.” If we accept that generally we have knowledge of the contents of our utterances, then, if S knows that her utterances of “brain” and “vat” mean brain and vat, it will be possible for S to know that she is not a brain in a vat. For if S has authority with respect to the contents of her utterances, then she can know that her utterance, “I am not a brain in a vat,” refers to brains and vats. On externalism, since S would not refer to brains and vats if she were a brain in a vat, she could know by mere introspection that she is not a brain in a vat. In fact, S can come to know many arbitrary propositions about the external world simply by attending to the contents of her thoughts. For if externalism is true, she can argue, then certain empirical conditions must be satisfied if she is even to have the thoughts about the external world that she does; since she knows what she is thinking, she knows these empirical conditions are satisfied. Thus, if S knows that she is thinking about brains and vats, then she can know that she is not a brain in a vat ; for if she was a BIV, then she would not be thinking about brains and vats, but instead about electrical impulses or some such thing. The standard BIV argument against scepticism, therefore, cannot be correct. This line of argument can then be used to conclude more generally that scepticism is false; for the argument can be run with virtually any utterance or thought with empirical content. If S knows that she is thinking about trees, for example, then she can know that there are trees.