I am very grateful to Peter Unger, Ernest Sosa, Ingmar Persson, Kadri Vihvelin, John Barresi, James Baillie, Stiv Fleishman and Xiao-Guang Wang, each of whom commented copiously and very helpfully on earlier written versions of this paper. I am grateful also for the many perceptive comments I received when I read an earlier version of this paper at Dalhousie University.