The Gifford Lectures Delivered in the University of St Andrews in 2003 These lectures were delivered in the University of St Andrews in April and May of 2003. It is difficult for me to find words to express my gratitude to the members of University of St Andrews for giving me the opportunity to deliver a series of Gifford Lectures in their university. Having attempted and discarded several more elaborate expressions of gratitude, I will say only that I am very grateful indeed for the honor they have done me. I am also grateful to many individual members of the university for all they did to make my stay in St Andrews a pleasant and productive one, and for their many acts of kindness to me and to my wife Lisette and my step-daughter Claire. Special thanks are due to Professor Alan Torrance, Dr Peter Clark (Head of the School of Philosophical and Anthropological Studies), Professor Sarah Broadie, and Professor John Haldane. I wish also to thank the audiences at the lectures for their insightful comments and questions, many of which I have responded to (however inadequately) in this book. These responses are to be found in the endnotes; in a few cases, they have taken the form of revisions of the text of the lectures. Finally, I thank the two readers to whom the Oxford University Press sent a draft of the manuscript of this book. I have tried to meet some of their concerns about particular passages (and I have responded to some of their more general comments and suggestions) in the notes and in the text.