 This account makes it straightforward to explain cases of a posteriori necessity: they are simply cases of sentences whose secondary intensions are necessary, but whose primary intensions are contingent. Suppose that ‘Hesperus’ and ‘Phosphorus’ were introduced, respectively, by the reference-fixing descriptions ‘the first celestial body visible in the evening (if it exists)’ and ‘the last celestial body visible in the morning (if it exists).’ Since the two descriptions single out the same object in the actual world, the sentence ‘If Hesperus exists, then Hesperus is Phosphorus’ is true in all worlds considered as counterfactual, and therefore has a necessary secondary intension. However, in some non-actual worlds, the two descriptions single out different objects. The sentence is false in such a world considered as actual. The primary intension of the sentence is therefore contingent.