  It seems that the binary relation first made its appearance in a 1951 article by Jonsson and Tarski, ‘Boolean Algebras with Operators’. In their Theorem 3.14 they established that every closure algebra is isomorphic to an algebraic system formed by a set and a reflexive and transitive relation between its elements; their Theorem 3.5 considered also a symmetry condition. In hindsight, these theorems (which explicitly concerned boolean algebras) can be viewed as in effect a treatment of all the basic modal axioms and corresponding properties of the accessibility relation. Concerning this article, Saul Kripke remarked (in Copeland 1996: 13):