Full Idea
Kripke's first (superior) route to necessary a posteriori has a sharp distinction between how the universe could conceivably and really be. ..On this picture conceivability is a fallible but useful guide, when combined with knowledge of actuality.
Gist of Idea
Kripke gets to the necessary a posteriori by only allowing conceivability when combined with actuality
Source
report of Saul A. Kripke (Naming and Necessity lectures [1970]) by Scott Soames - Significance of the Kripkean Nec A Posteriori p.168
Book Reference
Soames,Scott: 'Philosophical Essays 2:Significance of Language' [Princeton 2009], p.168
A Reaction
[compressed from p.168 and 170] To dismiss conceivability is ridiculous (see Williamson on that), and this formula of Soames sound right. To understand possibility, you have to study actuality (across time and space). Study history!