This little book is based on a course of four lectures which I had the honour of delivering in May 1963 in the Queen's University, Belfast, on the invitation of the Wiles Foundation. The lectures are printed substantially as they were spoken, save for a few additions and corrections. They were addressed to a general audience, and I hope that in their printed form they will be of interest to the general reader who has no specialised knowledge of ancient thought or of Christian theology. I have, however, supplemented them with footnotes which specify the evidence on which my statements are based, and develop some additional arguments and speculations.

My thanks are due in the first place to the Wiles Foundation and to all those who took personal trouble to make my visit to Belfast an agreeable experience: in particular to Dr Michael Grant, Vice-Chancellor of the Queen's University, and to Mrs Grant; to Mrs Austen Boyd; and to Professor Michael Roberts. I am most grateful also to those scholars who attended my lectures as guests of the Foundation and discussed them with me at the colloquia which followed, namely A. H. Armstrong, H. Butterfield, Henry Chadwick, R. Duncan-Jones, Pierre Hadot, A. H. M. Jones, A. D. Momigliano, H. W. Parke, Audrey Rich, S. Weinstock and G. Zuntz. Here and there in this book they will, I hope, recognize echoes of their individual contributions.