In this highly individual, and truly novel, approach to theoretical reasoning in physics, the author has provided a course that illuminates the subject from the standpoint of real physics as practised by research scientists. Professor Longair gives the basic insights, attitudes, and techniques that are the tools of the professional physicist, in a manner that conveys the intellectual excitement and beauty of the subject. The book is intended to be a supplement to more traditional courses for physics undergraduates, and the author assumes that his readers already have some knowledge of the main branches of physics. As the story unfolds, much of the core material of an undergraduate course in physics is reviewed from a more mature point of view. This is not, in fact, a substitute for existing texts. Rather it goes beyond them by improving the student's appreciation of the subject.