This book represents the Birkbeck Lectures given in the University of Cambridge in the spring and summer of 1948. I have printed the lectures more or less as they were delivered, except that I have made certain additions and expansions. I must in the first place express my thanks to the Master and Fellows of Trinity College, Cambridge, who did me the honour of inviting me to deliver the Birkbeck Lectures. Next I wish to thank the friends who have helped me at various stages in the making of this book: Dr Daniel Callus, O.P., Professor V. H. Galbraith, Dr J. R. L. Highfield, Professor Aubrey Gwynn, S.J., Miss Beryl Smalley, and Fr Leonard Boyle, O.P., who have read the book in whole or in part in manuscript and have given me valuable advice and criticism; Mr A. B. Emden, who has helped me over the biographies of Oxford graduates; and Dr R. W. Hunt and Professor David Knowles, O.S.B., who have read the proofs. I must also thank the Master and Fellows of Corpus Christi College, Cambridge, and the Warden and Fellows of New College, Oxford, for permission to print extracts from their manuscripts. I am much indebted to the Syndics of the Cambridge University Press for undertaking to publish this book and to the staff of the Press for their skill and patience.