When I began to consider how I could best fulfil my promise to read you a paper, my difficulty in selecting a subject arose, not from the dearth of matter, but from the wide range before me, as I was told I might select any mathematical subject I liked. The subject of mathematics as a whole I at once put aside, as I felt it would be impossible for me in a short paper to say anything new about it. For the same reason my favourite subject geometry was unsuitable, and as I was brought up on the Elements of Euclid, anything I might have to say about geometry would probably be considered old-fashioned and antiquated.