Is Water Necessarily Identical to H 2 O?

The “scientific essentialist” doctrine asserts that the following are examples of a posteriori necessary identities: water is H2O; gold is the element with atomic number 79; and heat is the motion of molecules. Evidence in support of this assertion, however, is difficult to find. Both Hilary Putnam and Saul Kripke have argued convincingly for the existence of a posteriori necessities. Furthermore, Kripke has argued for the existence of a posteriori necessary identities in regard to a particular class of statements involving proper names. Neither Kripke nor Putnam, however, has argued convincingly that sentences containing syntactically complex terms or descriptive phrases can express a posteriori necessary identities. I will argue by way of a hypothetical example that ‘water is H2O’ does not express a necessary identity. My argument is unique in that it attacks the relevant sufficiency claim needed to underwrite this putative necessary identity.1 That is, even if we grant that water is necessarily composed of H2O, we should not accept that H2O necessarily forms water. I shall not challenge the claim that there are a posteriori necessary identities involving syntactically complex expressions or descriptive phrases. (There are rigid descriptions and, hence, true necessary identities expressed with them. First, utterly trivial ones such as ‘the first natural number = the first natural number’. Second, slightly less trivial ones such as ‘Hesperus = the thing with which Hesperus is identical’. Third, nontrivial a priori ones such as ‘12 = the sum of 5 and 7’. And, fourth, nontrivial a posteriori ones such as ‘the thing with which Hesperus is identical = the thing with which Phosphorus is identical’.) Rather, I wish to stimulate investigation into the scope of the relevant scientific-essentialist claims. My argument that ‘water is H2O’ does not express a necessary identity should call into question, for instance, whether heat is necessarily