On the Plurality of Worlds (1986) is a book by the philosopher David Lewis that defends the thesis of modal realism. "The thesis states that the world we are part of is but one of a plurality of worlds," as he writes in the preface, "and that we who inhabit this world are only a few out of all the inhabitants of all the worlds." It is not to be confused with cosmic pluralism. Content The book is divided into four chapters. Chapter 1: A Philosopher's Paradise Chapter 1 begins with an exposition of modal realism. Lewis proposes that possible worlds, including ours, are real concrete things that are isolated from each other. "There are no spatiotemporal relations at all between things that belong to different worlds," and adds, "Nor does anything that happens at one world cause anything to happen at another." He recommends a plurality of worlds because hypothesizing it is "serviceable," the familiar analysis of necessity as truth at all possible worlds being "only the beginning." Lewis shows that modal realism can be used to give coherent accounts of, among other things, modal logic and counterfactual conditionals. Modality turns into quantification ("Possibly there are blue swans if and only if, for some world W, at W there are blue swans"), and counterfactual conditionals turn into statements of fact about possible worlds ('If it were that A, then it would be that C' is true if and only if C is true at the selected A-world.) 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. Of both the plurality of sets and the plurality of worlds, Lewis holds that "[t]he price is right; the benefits in theoretical unity and economy are well worth the entities." Among the further benefits: If possible worlds help with counterfactuals, they will help with causation. Closeness of worlds can help spell out what it means for a false theory of nature to be close to the truth. Idealizations such as the ideally rational belief system are "among the theoretical benefits to be found in the paradise of possibilia." The content of knowledge and belief are given by epistemically and doxastically accessible worlds, respectively—that is, worlds that, for all you know/believe, are the world you live in. A property is the set of all its