 The foregoing arguments from Kripke lead to the conclusion that the relevant kinds are not characterized of necessity by the readily observable properties of their instances. They do not show what does characterize those kinds. Kripke takes the view that, in most cases, it is certain microstructural properties that determine the nature of these natural kinds, as evident in his endorsement of the identities: ‘water is H2O’; ‘gold is the element with atomic number 79’, ‘light is a stream of photons’. Such essential properties of kinds are features, therefore, that may be unknown even to competent users of a relevant kind concept. Consequently, these are essences that, when known at all, are known a posteriori.