This work was made possible by the generous support of the Andrew W. Mellon Foundation, through a Distinguished Achievement in the Humanities Award, and by the Center for Advanced Study in the Behavioral Sciences at Stanford University, where I was also supported by the Mellon Foundation. I would also like to thank the Philosophy Faculty at the University of Oxford for the invitation to deliver the John Locke Lectures, and All Souls College, Oxford, for hosting me as a Visiting Fellow during Trinity Term of 2006.