I am indebted to Jonathan Sinclair-Wilson of Cambridge University Press for suggesting that I write this book and for encouraging it on its way. I hope that it may prove something of a companion piece to my The Theory of Knowledge (Macmillan, 1971). At all events, the thanks that I expressed in that book to Birkbeck College students are due again here. I have lectured on metaphysics at the College for more years than I care to remember, and successive generations of students have criticized, probed and commented on the views that I have expressed, in the way that Birkbeck students are ever willing to do. My debt to them is vast and I shall always be grateful.

Parts of a draft of the book were also read at a seminar which I, together with Mrs Dorothy Edgington, gave at the College. I am grateful for many comments made by her and others at the seminar.

I owe a very special debt to Mr Ian McFetridge who kindly read the first draft of the book and who made comments and criticisms which have been invaluable. He has saved me from many mistakes. Apart from the criticisms which he made, I am very grateful to him for the encouragement that his remarks afforded.

Thanks are due to other colleagues and students for points which are too numerous to mention. I am grateful, finally, to Mrs Georgia Wyver and Miss Maureen Cartwright for secretarial assistance of many kinds and for general help with the book.