 Many people have commented on earlier versions of this article offering valuable suggestions and criticisms, and I thank them all. They are Staffan Angere, Luc Bovens, Igor Douven, Branden Fitelson, David Glass, Michael Hughes, Lydia McGraw, Luca Moretti, Stefan Schubert, Tomoji Shogenji, Mark Siebel, Wolfgang Spohn, Paul Thagard and Greg Wheeler. Finally, I am indebted to an anonymous referee for a number of stylistic and other improvements. 