Full Idea
The necessary a posteriori crudely divides into two groups - (synthetic) identity statements (between rigid designators), and statements of essential properties. The latter is either statements of property identity, or of the essences of natural kinds.
Gist of Idea
The necessary a posteriori is statements either of identity or of essence
Source
Alan Sidelle (Necessity, Essence and Individuation [1989], Ch.2)
Book Reference
Sidelle,Alan: 'Necessity, Essence and Individuation' [Cornell 1989], p.25
A Reaction
He cites Kripke's examples (Hesperus,Cicero,Truman,water,gold), and divides them into the two groups. Helpful.