Full Idea
Kripke claims that all of his examples of the necessary a posteriori have the characteristic that we can know a priori that if they are true, they are necessarily true.
Gist of Idea
Kripke says his necessary a posteriori examples are known a priori to be necessary
Source
report of Saul A. Kripke (Naming and Necessity lectures [1970], 159) by Penelope Mackie - How Things Might Have Been 1.4
Book Reference
Mackie,Penelope: 'How Things Might Have Been' [OUP 2006], p.9
A Reaction
That is, it seems, that they are not really necessary a posteriori! The necessity seems to only arrive with the addition of a priori judgements, thus endorsing the traditional view that necessity is only derivable a priori. Hm.