  First, I thank two anonymous reviewers for the Stanford Encyclopedia  of Philosophy, whose pungent criticisms contributed to the clarity,  breadth and depth (and also the length) of this article. I thank my  friends, adversaries and colleagues (you know who you are) for their  criticism and encouragement. Finally, and most fervently, I thank the editor of the Encyclopedia, Ed Zalta, for patience and  understanding that go way beyond the saintly.