  In 1965 Prior visited California for several months, as Flint Professor of Philosophy at UCLA. For the first time he found himself among a group of enthusiasts for tense logic. Shortly after the visit ended he was to write: ‘I suppose that California is the most logically mature place in the world, and now that the logic of tenses is pursued so widely and so vigorously there, its raw pioneering days can be considered over’ (1967a: vi). When Prior arrived at UCLA, Nino Cocchiarella was just completing a Ph.D. thesis on quantified modal and tense logic, under Montague’s supervision (‘Tense and Modal Logic: A Study in the Topology of Temporal Reference’). Cocchiarella’s interest in the philosophy of time had initially been aroused by Reichenbach’s work on space and time, but it was his acquaintance with Prior’s Time and Modality that swept him into tense-logical research. (Only later did he learn of Scott’s work.) Prior’s visit also coincided with Hans Kamp’s arrival at UCLA as a graduate student. Kamp attended Prior’s lectures on tense logic in his first semester and became deeply interested in the subject. The lectures led more or less directly to the topic of Kamp’s Ph.D. thesis, written under Montague’s supervision and entitled ‘On Tense Logic and the Theory of Order’ (1968). In Kamp’s work the development of tense logic achieved a new level of formal sophistication. Segerberg, too, had just arrived in California, to study under Scott at Stanford. Segerberg had become interested in tense logic in Finland in 1964, at a series of summer seminars given by von Wright, who was independently pursuing a tense logic arising from his study of the logic of action, and later shown to be equivalent to a system Prior had discussed in Time and Modality (Prior 1957: 23–4; see von Wright 1965 and Segerberg 1967, 1989).