For several years I have enjoyed teaching an introductory college course that explored many of the issues discussed in this book. My thanks go out to all those students who have helped me think through these issues, who have challenged me to find better ways of explaining the material, and who have helped me see which aspects were of greater or lesser relevance. Several friends and colleagues were extremely generous with their time and talents, reading through large sections of the book, and providing wonderful feedback that greatly improved the book. Thanks in particular to Bob Schroer, Justin Sytsma, Matt Lund, Dave Hilbert, Nick Huggett and Bob Fischer. Parts of the book were presented to audiences in Durham, Leeds and Bristol. I am very grateful to those who made these events possible, and to those who attended and offered helpful questions and discussion. An anonymous referee from Cambridge University Press made several excellent suggestions that I'm sure have made the book better. My editors, Hilary Gaskin and Rosemary Crawley, were incredibly helpful with the book's preparation, for which I am very grateful. My thanks are also owed to the College of Arts and Science at East Tennessee State University, for awarding me a Summer Research Fellowship in 2014, which helped in the final stages of writing. My sister and parents have always provided enormous encouragement and support. There are many reasons why this book wouldn't have been written if it wasn't for them. Finally, my wife is a perennial source of inspiration and optimism. With respect to this project she was always willing to offer support, advice and reassurance. For the many ways she enriches my life I am indebted.