Whilst the greatest effort has been made to ensure the quality of this text, due to the historical nature of this content, in some rare cases there may be minor issues with legibility. This book represents a series of Messenger Lectures at Cornell University, the purpose of which is stated in a note printed at the end. The foundation is not primarily a classical one. Indeed this is the first time in thirty-five years that the lectures have been allotted to the Classical Department, and it was impressed on the lecturer that they should be suitable for a general audience. At the same time the addition of notes will, it is hoped, give the work a certain interest for the classical student and scholar. They contain not only references to passages supporting the text, but also sup plementary references and quotations, expansion of points which could only be summarily mentioned in the lectures, and a certain amount of additional information. Inevitably, however, the occa sion of the work has imposed severe limitations.