This project owes its inception to an invitation from the British Centre for Durkheimian Studies in Oxford to participate in a conference on Durkheim's The Elementary Forms of Religious Life in 1995. It was the paper that I presented there that first got me thinking about the social functions of the categories. I am especially grateful to Bill Pickering for his encouragement and continued interest in my work, as well as to Nick Allen and Willie Watts Miller, his coeditors for the proceedings volume that resulted from that conference. Throughout this and three other book projects with Bill in which I have been involved as either an author or a coeditor, I have had the opportunity to try out some of the ideas in this volume. Bill is one of the kindest, most generous people in academics with whom I have ever worked.

I am also deeply indebted to the Pittsburgh Center for Philosophy of Science, which hosted me during my sabbatical year in 1996–7, as well as to my family for allowing me to take them away from their comfortable home in Oak Park, Illinois, to live in Pittsburgh for a year. I would also like to thank the Illinois Institute of Technology for granting me this sabbatical year. I was able to conduct much of the research for this book and some of the initial writing during my stay in Pittsburgh.