 The semantic Twin Earth argument now proceeds by asking what the extension of ‘water’ is, as used by some Earthling, and likewise what the extension of ‘water’ is, as used by some Twin Earthling. Putnam’s appeal to our linguistic intuitions is supposed to yield the verdict that the extension of the Earthling’s use of ‘water’ includes all and only H2O whereas the Twin Earthling’s use of ‘water’ includes all and only XYZ. In support of this we may consider that this is the judgment we must come to if we Earthlings are not to deny what Earth chemists tell us, viz. that water is H2O, nor, symmetrically, deny the Twin Earth chemists’ assertion, ‘water is XYZ’. Nor will it do to gainsay our chemists’ judgment that water is H2O, given the assumption of the existence of XYZ. For that is tantamount to the suggestion that (actual) Berzelius (the discoverer of the correct chemical formula for water) was not entitled to assert that water is H2O until he had ruled out the possibility of some substance other than H2O sharing water’s manifest qualities.