Most of the work for this book was done while I held a research studentship at Christ's College, and the first debt which I have to acknowledge is, therefore, one of gratitude to the Master and Fellows. I should like to thank the Managers of the Ellen McArthur Fund for their permission to make slight alterations to chapters i to vii as originally submitted to them, and to add chapters viii to xi. Others to whom my grateful thanks are due are Mrs J. V. Robinson and Mr P. Sraffa, who have given me valuable criticism and advice, and enabled me to correct a number of mistakes (though the responsibility for the mistakes which remain is, of course, entirely my own); the staff of the University Press who, in spite of their own difficulties at this time, have always been willing to help with mine; and my wife, who has given me constant assistance from the preparation of rough notes to the compilation of the index.