 The thought experiment is now almost in a position to claim the foregoing as a counterexample to descriptivism, for XYZ will satisfy any set of manifest properties that could be constitutive of the meaning of ‘water’ as used on Earth. Yet the extension of that term excludes XYZ. However, the reference to chemists’ knowledge made above reminds us that there is a difference between not only the physical constitution of Earth and Twin Earth, but also a difference between the psychological states of their inhabitants. For (some of) those on Earth believe ‘water is H2O’ whereas their Twin Earth counterparts believe ‘water is XYZ’. Perhaps, at least for those speakers, those beliefs are also constitutive of the meanings of the two terms ‘water’ (Earth) and ‘water’ (Twin Earth). In order to exclude this response, Putnam asks us to consider the same comparison made in 1750, before any beliefs of this form were held. We now assume that there has been no major change in the (cross-temporal) extension of ‘water’ (Earth) since 1750. (That is to say, when David Hume asked for a glass of what he called ‘water’, he was requesting the same stuff as we are when we make a request using the same term.) Since, as argued above, the extensions of ‘water’ (Earth) and ‘water’ (Twin Earth) differ today, their extensions differed in 1750. But in 1750 the descriptions associated with the word ‘water’ cannot differ between Earth and Twin Earth. This is because the molecular structure of water was unknown in 1750, prior to the chemical revolution (when phlogiston theory was replaced by Lavoisier’s new oxygen theory). Given that the molecular structure was unknown even to the experts, the descriptive resources available to Earthians and their Twin Earthian counterparts were identical. Nonetheless, their references using ‘water’ differed. Hence, descriptions do not suffice to fix reference.