This book is a defense of modal realism; the thesis that our world is but one of a plurality of worlds, and that the individuals that inhabit our world are only a few out of all the inhabitants of all the worlds. Lewis argues that the philosophical utility of modal realism is a good reason for believing that it is true. About author(s): David Lewis (1941- 2001) was Professor of Philosophy at Princeton University. His publications include Convention (reissued by Blackwell 2002), Counterfactuals (reissued by Blackwell 2000), Parts of Classes (1991), and of numerous articles in metaphysics and other areas. Many of his writings are available in his Collected Papers .