 This is a separate topic from that of the previous section, since necessity and the a priori can come apart. Consider the claim that the nucleus of an atom of gold contains 79 protons. This claim is not a priori. It is the fruit of an empirical inquiry into the nature of gold, and could certainly not have been deduced from the meaning of “gold”. It is, however, a necessary truth (Kripke 1980: 123–125). The nature of gold is, in this respect, immutable. If you added or subtracted protons from the nucleus, you would no longer have an atom of gold, but some other element instead.