 It is important for this example that, although 1750 residents of Earth did not yet realize this, all water was in fact H2O. Had it turned out instead that some of the stuff called “water” was H2O while other stuff called “water” was XYZ, then water would not have turned out to be a single “natural kind.” In that case the English word “water” would refer to anything that was either H2O or XYZ, and we could say that the Earthly and Twin-Earthly ‘water’-words referred to the same thing. Again, if it had turned out that there were a huge number of different microstructures that produced the observable properties of water, then water would not have been a natural kind at all. In that case we would probably say that anything with the right observable properties was water, and in that case again we could say both ‘water’-words were coreferential. In fact, though, neither of these possibilities obtains. Water is a natural kind whose essential nature is that it has the chemical structure H2O; since there is no H2O on Twin Earth, there is no water there. Twin Earthlings never had occasion to give a label to water, since there is none on their planet, so their word “water” does not refer to water.