Abstract

SCIENTISTS HAVE DISCOVERED that water is H2O. Water is H2O is true. But is it a necessary truth? In other words, is it true in all possible worlds? Some people think it is. For example Hilary Putnam, in his well-known Twin Earth argument, concludes that "water is H2O" is necessarily true; thus a liquid which phenomenally resembles H2O and fits the description of water in almost all aspects, but has the chemical formula XYZ, cannot be water. Saul Kripke has made a similar claim about the necessary identity between water and H2O. Because this type of truth is based on empirical discoveries, Kripke calls truths of this sort "necessary a posteriori." The thesis shared by Putnam and Kripke has two premises: a realist view that natural kinds exist independently of human cognition, and a theory of direct reference of natural kind terms. Opposing the view that natural kind terms pick out objects through descriptions, Putnam and Kripke hold that natural kind terms pick out natural kinds in the world in a direct way. Based on these two premises, they argue that, if two natural kind terms, A and B, designate the same thing in the world, "A = B" expresses a necessary identity.