I would like to express my deepest gratitude to George Myro for his wonderful insight, advice, and support, in writing this paper. I would also like to thank Ermanno Bencivenga, John Broome, Janet Broughton, Alan Code, Paul Grice, Mark Johnston, David Lewis, Paul Kube, Dugald Owen, Steve Yablo, and Colloquium participants at University of Virginia and Ohio State University for their very helpful discussions.