When I started writing this book, I realized for the first time how long I had been thinking about its major themes. So I should like to thank in retrospect many teachers, friends and colleagues both inside and outside the academic world who have helped me to consider these issues. More specifically, I am especially grateful to Jim Secord for his invaluable comments on a first draft, and also to Michael Fara, Stephen Gaukroger, Michael Hoskin, Sarah Hutton, Rebecca Stott and Judith Zinsser for their constructive criticisms of individual chapters. In addition, many thanks to my copy editor Mandy Greenfield, and also to my agent David Godwin and my editor at Pimlico, Will Sulkin, without whom this book would never have been published.