 Above we saw that an identity statement conjoining two rigid designators expresses a necessary truth, one that may be knowable only a posteriori. That follows simply from the definition of ‘rigid designator’. We have also seen that natural kind terms are rigid designators. That seems simply to be a fact about the semantics of natural kind terms. Consequently it looks as if a non-trivial metaphysical fact—that there are necessary truths knowable only a posteriori—is a consequence of Kripke’s semantics for natural kind terms. Furthermore, when we consider theoretical identities, such as ‘water is H2O’ and ‘gold is the element with atomic number 79’ etc., it appears that we are being given a special kind of necessary truth, a truth that it necessary because it specifies the essence of a kind.