 Both Putnam and Kripke take arguments of this sort to generalize to natural kind terms. The best known of the semantic arguments as applied to natural kind terms is Putnam’s Twin Earth thought experiment (see Section 1.3.1). Putnam asks us to imagine that elsewhere in the universe there is a planet exactly like Earth in virtually all respects, which we may call ‘Twin Earth’. However, where on Earth we find H2O, on Twin Earth we find XYZ, a substance with a fundamentally different chemical structure from H2O, but which shares all its manifest properties (boiling and freezing points, taste, odour, ability to support life, being the predominant constituent of precipitation, seas, lakes etc.) with H2O. Every inhabitant of Earth has a twin equivalent on Twin Earth. Twin Earthlings (at least those in the parts bearing a geographical similarity to Earth’s United States, Australia, Jamaica, Pitcairn etc.) speak a language rather like English and use the term ‘water’ in circumstances precisely analogous to our uses of ‘water’ on Earth, e.g., they say that their lakes and reservoirs are full of ‘water’.