In the summer of 1960 the author was at Oxford, just finishing his degree examinations in chemistry and wondering how to spend his fourth year – Oxford being then a pioneer in the system of requiring from science undergraduates a year’s work on a research project before graduation. Science was not some disinterested and isolated search after truth, free of metaphysics and manifestly a good thing in a naughty world: it was one human activity among others, not an austere model of rationality. Students taking this course, coming mostly from science or from philosophy, had to wrestle with context; and they mostly enjoyed it. They had to try to get inside the beliefs of people from a past that was very different from the present, and yet had led inexorably to it. Beliefs are not abstract and untestable convictions, but concern how things should be done.