  In 1956 Prior wrote up (and circulated in mimeograph form) his and Meredith’s formal work on what he later described (1962a: 140) as the ‘logic of world-accessibility’, in a paper entitled ‘Interpretations of Different Modal Logics in the “Property Calculus”’ (Meredith and Prior 1956, first published in Copeland 1996). It carried the attribution ‘C.A.M., August 1956; recorded and expanded A.N.P.’. Prior mentioned this paper in Past, Present and Future (1967: 42–5) and in his 1962 articles ‘Possible Worlds’ and ‘Tense-Logic and the Continuity of Time’. The paper seems to have been the earliest to employ a binary relation between points in order to discriminate between S5 and weaker systems, the points being regarded as possible states of affairs. Carnap’s pioneering 1946 paper concerned only S5 and contained no such relation.