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.

In this paper, I will defend modalfictionalism. The paper has two parts. In thefirst part, I will suggest a revised version ofmodal fictionalism which can avoid certaintechnical problems. In the second part, I willpropose a nominalized version of modalfictionalism and a general scheme offictionalism for the nominalist.

Modal realism -- Time, space, world -- Existence -- Actuality -- Modal realism and modal tense -- Transworld individuals and their identity -- Existensionalism -- Impossibility -- Proposition and relief -- Fictional worlds -- Epistemology.