Full Idea
An identity statement between names (such as 'Hesperus' and 'Phosphorus'), when true at all, is necessarily true, even though one may not know it a priori.
Clarification
Hesperus (the evening star) and Phosphorus (the morning star) turned out to be the planet Venus
Gist of Idea
"'Hesperus' is 'Phosphorus'" is necessarily true, if it is true, but not known a priori
Source
Saul A. Kripke (Naming and Necessity lectures [1970], Lecture 3)
Book Reference
Kripke,Saul: 'Naming and Necessity' [Blackwell 1980], p.108
A Reaction
This seems correct, but one should not read too much into it. What should we say if Venus fissions into two, one for the morning, one for the evening? That identity implies x=x doesn't prove the existence of unchanging essences.