 The author would like to thank Hossein Ameri, Tim Bayne, Rich Cameron, Brian Garrett, Robert Pasnau, Stewart Saunders, Jessica Wilson, and particularly David Chalmers for their help in constructing this entry. In addition, the author and editors would like to thank two readers, Joshua R. Stern and Greg Stokley, for discovering numerous typographical errors on an earlier version. Their volunteer efforts were entirely unsolicited and very much appreciated.