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.

E. J. Lowe; The problems of intrinsic change: rejoinder to Lewis, Analysis, Volume 48, Issue 2, 1 March 1988, Pages 72–77, https://doi.org/10.1093/analys/48.2.7.