 Now, Putnam’s response to the skeptic is to argue that though we cannot rule out the brain-in-a-vat hypothesis on the grounds of how things appear to us, we can rule it out nonetheless. How? Putnam’s claim is that we can rule it out because on a plausible theory of reference, it is self-refuting: that is, ‘I am a brain-in-a-vat [or BIV, for short]’ cannot be truly affirmed by anyone. The theory of reference Putnam uses as a premise is a causal one, which states that ‘one cannot refer to certain kinds of things, e.g., trees, if one has no causal interaction at all with them, or with things in terms of which they can be described’ (Putnam 1981: 16–17). Putnam defends this theory, on the grounds that it alone can explain how reference occurs in a way that is not ‘magical’, i.e., which does not assume that the connection is just somehow intrinsic between representations and their referents. It then follows, according to Putnam, that a BIV affirming ‘I am a BIV’ is saying something true only if the BIV is in what the BIV is calling a ‘vat’. But the BIV is not in one of those; rather, he is in a real vat. What he is calling a ‘vat’ is that to which his use of that term causally connects in a referential way—where for a BIV, that is something in the computer that prompts his applications of ‘vat’ by giving him hallucinations of the appearance of vats. Yet the BIV is not in that part of the computer. Thus, Putnam concludes, ‘I am a BIV’ cannot be truly asserted by anyone, much like ‘I do not exist’ or ‘I cannot construct a meaningful sentence’ (cf. Bardon 2005). It therefore cannot ever be wrong to assert that one is not a BIV, so that in this sense ‘I am not a BIV’ is an incorrigible claim (cf. Putnam 1981: 14–15). Putnam therefore holds that we can rule out the BIV hypothesis on a priori grounds, and thus refute the skeptic.