Full Idea
People think 'necessary' and 'a priori' mean the same for two reasons: we can assess what is feasible in all possible world by running them through our heads, and something known a priori avoids looking at the world, so it must be necessary.
Gist of Idea
A priori = Necessary because we imagine all worlds, and we know without looking at actuality?
Source
Saul A. Kripke (Naming and Necessity lectures [1970], Lecture 1)
Book Reference
Kripke,Saul: 'Naming and Necessity' [Blackwell 1980], p.38
A Reaction
[compressed] Kripke denies this doctrine, and pulls the concepts apart. Kant seems to be the chief representative of the view he is attacking. Hossack defends the older view.