Full Idea
The Kantian rationalist view is that what is necessary is always knowable a priori, and what is knowable a priori is always necessary.
Gist of Idea
Necessity is always knowable a priori, and what is known a priori is always necessary
Source
report of Immanuel Kant (Critique of Pure Reason [1781]) by Laura Schroeter - Two-Dimensional Semantics 2.3.1
Book Reference
'Stanford Online Encyclopaedia of Philosophy', ed/tr. Stanford University [plato.stanford.edu], p.22
A Reaction
Nice to get a clear spelling out of the two-way relationship here. Why couldn't Kant put it as clearly as this? See Kripke for the first big challenges to Kant's picture. I like aposteriori necessities.