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.

Most of the major schools of contemporary philosophy, from Marxism to Existentialism, are reactions to Hegelianism and all, if they are to be understood, require some understanding of Hegel's Logic. From its first appearance in 1812, this work has been recognized by both admirers and detractors alike as being the absolute foundation of Hegel's system.