This book has taken a long while to complete, and has accrued many debts. In particular, I mention Professor Antony Black and Dr P. J. Fitzpatrick who, as always, shared their scholarship with me without stint. I mention also the editorial staff of the Cambridge University Press, and above all Virginia Catmur, to whose amazing skills as a copy-editor this volume, like my edition of Augustine's De civitate Dei in the same series, owes so much. Finally, I thank my colleagues and students at the University of Durham for their unfailing support, encouragement and stimulation.