  At almost exactly the same time, Prior was reading Kripke’s first paper, ‘A Completeness Theorem in Modal Logic’ (Kripke 1959a), in his capacity as referee for the Journal of Symbolic Logic. In this paper Kripke stated and proved a completeness theorem for an extension of S5 with quantifiers and identity; the binary relation made no appearance. His first publication to mention the binary relation—which he interpreted as a relation of relative possibility between worlds—was written in 1962 and appeared in 1963 (Kripke 1963); the paper contained completeness proofs for propositional \(M\), S4, \(B\), and S5. Kripke reported (in correspondence with Copeland) that the idea of the binary relation occurred to him much earlier than 1962, in fact shortly after his paper on S5 was first submitted in the spring of 1958.