 Putnam’s goal is to refute the skeptical argument that is based on the BIV hypothesis, by providing a counterargument for the conclusion that we are not BIVs. Thus, each of us is provided with a way of knowing that she is not a BIV, contrary to premise (3) of the skeptical argument (SA) above. The crucial move in Putnam’s argument is a sub-argument to the effect that that if one were a BIV, one would not be able to grasp the meanings of various general terms, such as ‘tree’, ‘brain’, and ‘vat’. For in order for an utterance of ‘tree’ for example to represent or refer to trees, Putnam maintains, there must be some causal connection between uses of ‘tree’ and real trees (Putnam 1981 [1999: 37]). Following Wright (1992: 71), I will call this ‘the causal constraint’. A similar point, Putnam assumes, holds for the words ‘brain’ and ‘vat’, as well as for mental utterances of, and the concepts expressed by, such words. It follows that if we are BIVs, then since our uses of ‘brain’ and ‘vat’ do not bear the right causal relations to actual brains and vats, we cannot so much as think or say that we are brains in a vat (Putnam 1981 [1999: 31]).