These lectures were delivered at the University of Aberdeen in 1960. Since then, they have been extensively revised. It soon became obvious that many changes were needed. For example, in Series I far too little had been said about the relation between belief and evidence, and the important and difficult subject of the evidence of testimony had been neglected. Again, in Series II nothing at all had been said about Newman's celebrated distinction between ‘notional’ and ‘real’ assent, though his criticisms of Locke's doctrine of Degrees of Assent had been discussed, perhaps at excessive length, in Series I, Lecture 6. The result has been that many of the twenty lectures are now far longer than any orally-delivered lecture could possibly be. Nevertheless, I have thought it proper to preserve the original lecture form as well as I could, and have tried to write throughout as if I were addressing a visible audience.

I wish to express my warm gratitude to the University of Aberdeen for the very great honour it did me in inviting me to deliver the lectures, and for all the kindness I received during my two very happy periods of residence there. I also wish to thank my friend and former pupil, Professor Jonathan Harrison of the University of Nottingham, most warmly for all the help he has given me in preparing the Lectures for publication. I am greatly indebted to him for many valuable comments and suggestions, and for most kindly making arrangements to have the manuscripts typed. Indeed, without his help, and his encouragement too, the Lectures might never have been published at all.

Finally, I hope that I may be allowed to pay my pious tribute to the munificent and far-sighted Founder of these Lectureships, and that my subject, the Epistemology of Belief, has at any rate some connection with the topics which Lord Gifford wished his Lecturers to discuss. Yet anyone who considers the great and good men who have delivered Gifford Lectures in the past—William James, for example, or Samuel Alexander, to name only two—must wonder how he can possibly live up to the standards they have set for their successors. I myself have felt like a humble barn-owl, who is expected to soar in the noon-day sky as if he were an imperial eagle; but all he can do is to flit slowly in the winter twilight just above the hedgerows, searching carefully for very small field-mice which may or may not be there.