  Prior’s detailed contributions to the development of modal logic are legion. At least one aspect of his work has not received the recognition it deserves. Several years ahead of Kripke, Prior and his collaborator Carew Meredith invented crucial elements of the possible worlds semantics for propositional modal logic, including the all-important binary relation, which opens the way to modelling systems of different strengths. Meredith was a lecturer in mathematics at Trinity College, Dublin; his interest in logic had been stimulated by the arrival of Łukasiewicz in Dublin shortly after the second world war.