On the Plurality of Worlds (1986)[1] is a book by the philosopher David Lewis that defends the thesis of modal realism.

He argues that the theoretical utility of modal realism provides a reason to accept it as true, drawing a parallel to the fruitfulness of set theory in mathematics.

Among the further benefits: Lewis begins his attack on "ersatzism" in this chapter, rejecting Quine's suggestion that a possible world might be taken as a mathematical representation giving the coordinates of the spacetime points that are occupied by matter.

Chapter two contains Lewis's response to several arguments, many set theoretic in nature, that attempt to show that modal realism flounders because of the quantity of worlds that must be postulated.

These views are attempts to develop a theory that provides the explanatory power Lewis notes in chapter one without modal realism's ontological commitment to an infinitude of concrete possible worlds.