If I use the name 'Hesperus' to refer to a certain planetary body when seen in a certain celestial position in the evening, it will not therefore be a necessary truth that Hesperus is ever seen in the evening. That depends on various contingent facts about people being there to see and things like that. So even if I should say to myself that I will use 'Hesperus' to name the heavenly body I see in the evening in yonder position of the sky, it will not be necessary that Hesperus was ever seen in the evening. But it may be a priori in that this is how I have determined the referent.

Naming and Necessity (1980, p. 291)