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.

This is the second edition of an award-winning 1981 book (Princeton University Press and Basil Blackwell, based on the author’s doctoral dissertation) considered to be a classic in the philosophy of language movement known variously as the New Theory of Reference or the Direct-Reference Theory, as well as in the metaphysics of modal essentialism that is related to this philosophy of language.

Terence Parsons, Peter Woodruff; X*—Worldly Indeterminacy of Identity, Proceedings of the Aristotelian Society, Volume 95, Issue 1, 1 June 1995, Pages 171–192.