This book was written in the spring and summer of 1968, and submitted as a doctoral dissertation at the University of Cambridge. With its completion I left philosophy and descended to less aethereal regions. Learning, and now teaching, a new subject has meant that I have had to keep revision to a minimum, and have been unable to take account of recent additions to the literature. I hope that, despite this, some of the ideas contained here may be of use to those who persist in the struggle I have fled.

Nowhere is originality more difficult to assess than in philosophy. Some specific debts are easy to acknowledge, resulting as they do from the conscious borrowing of an account, or theory, or idea. I owe such debts to D. Davidson, for the account of singular causal statements outlined in Chapter 11, and to H. P. Grice, for certain ideas which I use in Chapter XVIII, and for the method of approach which governs that and the preceding chapters. Indeed my debt to these philosophers is much greater than the text may suggest, for the book is in large part an elaboration in a particular area of views they have espoused.

Other specific debts are less easily acknowledged, for they are unknown to or forgotten by the debtor. Any philosophical discussion is a co-operative enterprise, which leaves the participants enriched by new ideas of indeterminate authorship. Inevitably, therefore, some of this book will be derivative, but how much and from whom is impossible to say.