Any necessary truth, whether a priori or a posteriori, could not have turned out otherwise.

T is valid in any reflexive frame [\langle WR\rangle]: if [w\Vdash \Box A] then [w\Vdash A] since w R w. Another of his most important contributions is his argument that necessity is a 'metaphysical' notion which should be separated from the epistemic notion of a priori and that there are necessary truths which are a posteriori truths such as "Water is H2O. A Kripke frame for a language with [\{\Box_i\mid\i\in I\}] as the set of its necessity operators consists of a non-empty set W equipped with binary relations Ri for each i ∈ I.

" His most famous work is Naming and Necessity (1980). Saul Aaron Kripke (born November 13 1940) is an American philosopher and logician.