instances in all worlds where the property obtains, and similarly for relations. Propositions are properties instantiated only by entire possible worlds, so a proposition is a set of possible worlds, the worlds at which it holds or is true. Modal realism is favored over traditional systems of modality because it does not involve primitivity or brute modal facts. Modality can be reduced in the Lewisian account, but in traditional systems modal quantifiers, such as "possibly", are defined in terms of other modal quantifiers or notions. It has been argued however, that Lewis treats the existence of worlds as primitive or brute and explains modality in terms of that analysis. 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. No possible world is identical with any Quinean ersatz world, but for every such ersatz world there is a genuine possible world. Chapter 2: Paradox in Paradise? 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. Chapter 3: Paradise on the Cheap? In chapter three, Lewis considers various views that he calls "ersatz modal realism." 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. Lewis argues that all of these attempts fail but that each fails for different reasons. Chapter 4: Counterparts or Double Lives? Finally, chapter 4 contains Lewis's development of counterpart theory. According to Lewis's theory, each actual object exists only in the actual world and not in any non-actual world. However, each actual object has counterparts in infinitely many possible worlds that are not identical to the actual object, but that "resemble [it] in important respects." References 1986 non-fiction books Philosophy books