THE idea of collecting these essays occurred to me when, in the leisure of retire- ment, I scanned some of my own books and found that two of the more widely read show a startling change of attitude to some of the fundamental concepts of science. These are Einstein's Theory of Relativity of 1921 and the American edition of The Restless Universe of 1951. I have taken the introduction of as the former the first item of this collection, the postscript...