  Ayer’s rejection of these kind of de re necessities was, at root, a consequence of his epistemological approach to their evaluation. The senses of terms, he thought, were dependent on their associated descriptions, these being dependent on what we knew of their reference, and these senses accounted for the presence or absence of necessity. So it is identity of sense in “Hesperus is Hesperus” that makes that sentence necessarily true, whilst absence of sense-identity renders “Hesperus is Phosphorous” contingently true. He rejected the thought that natural kind terms have their sense fixed by their internal constitution on the grounds that many, if not most, users of natural kind terms are ignorant of the nature of the relevant internal constitutions. In this he is in agreement with those who recognize the kind of necessity arising from primary intensions (Chalmers 2004) or ‘A intensions’ (Jackson 1998), such intensions being determined by (some) phenomenal properties of the kinds referred to.