Lectures of this kind may be either written up or written down. Like most of my predecessors in the BBC’s Reith Lecturership, I have chosen the latter course, and what may be read here (with a few trivial alterations) is what I actually said during the broadcasts on Sunday evenings in the winter of 1959. The Notes, addressed to a professional audience, are gathered together at the end of the text; I felt that if they were in the text itself they would interrupt the flow of the arguments.