I am grateful to the following colleagues for their helpful suggestions on earlier drafts of this entry: Maria Bottis, Jeff Buechner, Lloyd Carr, Jerry Dolan, Fran Grodzinsky, Ken Himma, Larry Hinman, Lundy Lewis, Martin Menke, Richard Spinello, and Michael Zimmer. I would also like to thank the anonymous SEP reviewers for their insightful comments and helpful suggestions.