 ‘Cicero = the organism descended from sperm s and egg e’ shows that statements about individuals can, in effect, be theoretical identity statements. The most commonly discussed theoretical identity statements concern kinds. They include ‘Gold = the element with atomic number 79’ and ‘Water = H2O’ (Kripke 1980, pp. 138–40, 148). Similar reasoning might apply. Here the reasoning is again nearly always left implicit; one must fill in gaps to present a general line of thought that seems to accord with a widely held, if not widely articulated, tradition. For that tradition, in order for it to be the case that ‘Water = H2O’ is necessarily true, both designators must be rigid if either is. So if the statement is necessarily true and either ‘water’ is rigid by virtue of being a name for a substance or else ‘H2O’ is rigid by virtue of expressing an essence, both designators need to be rigid. And again, the rigidity of designators in a true theoretical identity statement is sufficient for the statement’s necessity. If ‘H2O’ is a rigid designator of a substance, and ‘water’ is a rigid designator of a substance, and ‘Water = H2O’ is true, then that statement is necessarily true.