  Prior would not have been completely surprised to learn how useful tense logic has proved to be in computer science. He himself took little interest in computing, beyond including material on elementary boolean circuit theory in his undergraduate lectures, but a number of the logicians with whom he was in touch were more deeply involved (Dov Gabbay and Dana Scott, for instance). Through others Prior knew something of the potential. He wrote ‘There are practical gains to be had from this study too, for example in the representation of time-delay in computer circuits’ (1996a: 46). In Past, Present and Future he remarked concerning logics of discrete time that their usefulness ‘does not depend on any serious metaphysical assumption that time is discrete; they are applicable in limited fields of discourse in which we are concerned only with what happens next in a sequence of discrete states, e.g. in the workings of a digital computer’ (1967a: 67). Other logics from a wide group that Prior and von Wright pioneered are also finding computational applications, for example epistemic logic in artificial intelligence and knowledge-base engineering, and the logic of action in programming theory. It is pleasant to reflect that two major forces in the genesis of these software technologies were Prior’s love of ancient and medieval logic and his concern to make conceptual room for freedom of the human will.