This book is the product of a way of presenting the subject which has been forming in my mind since I began to teach the subject a good many years ago. I am not the first nor, I suspect, shall I be the last to have found the writing of a ‘general’ book more of a challenge than I had anticipated when I took it on. Only the reader will be able to tell how useful and successful the experiment will have been.

The preface is usually the last part of a book to be written. By the time that stage is reached, the writer knows to whom he is indebted. To the many Liverpool students who, over the years, have stopped me in my tracks by telling me that what seemed clear to me was not so to them, I owe a debt of gratitude. I am grateful, too, to the Syndics of the Cambridge University Press for inviting me to write this book, to Mrs Betty Plummer for typing the text, and to Kay McKechnie for carrying out her work as the Press's subeditor with such efficiency.

To an old friend, James Sherborne, I owe a particularly warm word of thanks. A dozen or more years ago he organised a very successful conference at Bristol on the theme of this book. When I asked him if he would read my draft, he accepted and completed the work with speed.