  Prior’s earliest mention of a logic of time-distinctions is to be found in the penultimate chapter of his unpublished manuscript The Craft of Formal Logic. The Craft, completed in 1951 (Prior 1951), was intended to be his first book on formal logic. Following von Wright in ‘Deontic Logic’ (von Wright 1951b), Prior remarked that there are other groups of modal predicates to be set alongside the ordinary or ‘alethic’ modes of necessity and possibility. He referred to these non-alethic modalities as ‘quasi-modals’. After noting that Peter of Spain classified adverbial distinctions of time as modes, he said: