  Prior’s entry into modal logic was bound up with his passion for ancient and early logic. His first essay on the subject, the penultimate chapter—‘Modality’—of his ill-fated manuscript The Craft of Formal Logic, is largely historical in nature, with discussions of Aristotle, Peter of Spain, John Wallis, the Port Royal Logic, Isaac Watts’ Logick, Whately, Aldrich, de Morgan, and Hume and Mill on natural necessity. To judge by his references in The Craft, his first encounters with modern symbolic modal logic must have been the pioneering explorations by Lewis in his and Langford’s Symbolic Logic, Bochenski’s chapter ‘La Logique de la Modalité’ in his La Logique de Théophraste, and Feys’ article ‘Les Systèmes Formalisés des Modalités Aristotéliciennes’ (Lewis and Langford 1932, Bochenski 1947, Feys 1950). Other important early influences were von Wright’s ‘Deontic Logic’ and An Essay in Modal Logic, both published in 1951. Probably through Bochenski’s writings, Prior had discovered Łukasiewicz’s work in symbolic modal logic by the time of ‘In What Sense is Modal Logic Many-Valued?’. In that paper Prior focussed on Łukasiewicz’s ‘On Variable Functors of Propositional Arguments’ (Łukasiewicz 1951b), which he described in a 1952 review of Łukasiewicz’s symbolic logic as ‘quite the most exciting contribution that has been made to symbolic logic in English for a very long time’. Łukasiewicz’s work on modality was a major influence on Prior, who wrote in the Preface to Time and Modality: ‘[W]hile I differed radically from the late Professor Łukasiewicz on the subject of modal logic, my debt to him will be obvious on almost every page’.)