This paper largely emerged from extended discussions with Georges Rey. I am grateful to him for pressing me on the arguments and providing very helpful advice on several drafts. I am also grateful to John O'Leary Hawthorne, Barry Loewer, and Daniel Nolan, for reading and commenting on an earlier draft. Versions of this paper were presented at the University of Brisbane (1994), Wollongong University (1994), Monash University (1994), Johns Hop kins University (1995), Cincinnati University (1995), North Carolina State University (1995) and Rice University (1995). My thanks also to the many people at these talks who provided stimulating discussion.