 Given semantic externalism, the argument proceeds by claiming that if we were brains in a vat we could not possibly have the thought that we were. For, if we were so envatted, we could not possibly mean by ‘brain’ and ‘vat’ what unenvatted folk mean by these words since our words would be connected only to neural impulses or images in our brains where the unenvatteds’ words are connected to real-life brains and real-life vats. Similarly, the thought we pondered whenever we posed the question “am I a brain in a vat?” could not possibly be the thought unenvatted folk pose when they ask themselves the same-sounding question in English. But realism entails that we could indeed be brains in a vat. As we have just shown that were we to be so, we could not even entertain this as a possibility, Putnam concludes that realism is incoherent [Putnam 1981]. For this argument to work, however, Putnam must be assuming a rather restrictive form of modal rationalism: we could be brains in a vat only if in the circumstance that we were envatted, we could conceive that we were envatted.