Work on this paper began while I was at the University of Wisconsin-Madison and was supported in part by a Charles Phelps Taft Postdoctoral Fellowship at the University of Cincinnati. Versions were read at East Carolina University and the University of San Diego. For comments and discussion I am very grateful to Chris Gauker, Jagdish Hattiangadi, Russell Trenholme, Elliott Sober, Michael Byrd, Ernest LePore, and Dennis Stampe. I am also very grateful to an anonymous referee for this journal who provided welcome comments which led to improvements.