Full Idea
Kripke has demonstrated the existence of necessary truths such as "water is H2O" whose necessity is only knowable a posteriori.
Gist of Idea
Kripke has demonstrated that some necessary truths are only knowable a posteriori
Source
report of Saul A. Kripke (Naming and Necessity lectures [1970]) by David J.Chalmers - The Conscious Mind 2.4.2
Book Reference
Chalmers,David J.: 'The Conscious Mind' [OUP 1997], p.131