There is a widespread assumption that the classical work in philosophical semantics of Saul Kripke (1980) and Hilary Putnam (1975) has taught us that the essences of natural kinds of substances, such as water and gold, are discoverable only a posteriori by scientific investigation. It is such investigation, thus, that has supposedly revealed to us that it is an essential property of water that it is composed of H2O molecules. This is the way in which Scott Soames, in a recent (...) paper, makes the point in the case of water and H2O: [Here is] how a Kripkean explanation of the necessary a posteriority of [‘Water = H2O’] would go – or at any rate my version of it. The account holds that ‘water’ is a non-descriptive, directly-referential term designating a substance – where substances are taken to be physically constitutive kinds (instances of which share the same basic physical constitution). It is further assumed that a kind of this sort may have different instances in different world-states, and that if a and b are kinds with the same instances in all possible world-states, then a is b. These are clearly metaphysical assumptions, to which we add the natural corollary that for any substance, s, if, in some possible world-state, instances of s have a certain molecular structure, then instances of s have that structure in every world-state. In other words, we assume that it is an essential property of a substance that instances of it have the molecular structure they do. From this it follows that [‘Water = H2O’] is necessary if true, and that ‘H2O’– which I take to be equivalent to ‘the substance instances of which have a molecular structure with two hydrogen atoms and one oxygen atom’– is a rigid designator. Being true, [‘Water = H2O’] is, therefore, necessary. Since knowing the proposition it expresses requires knowing of a certain substance that its instances have a particular chemical structure, [‘Water = H2O’] is knowable only a posteriori. (Soames 2007: 36). (shrink)