 Second, he offers an account of modality, his famous “realism” about possible worlds. Lewis, like many philosophers, takes talk of possibility and necessity to be best explicated as disguised quantification over possible worlds (and possible inhabitants thereof), and he was endlessly ingenious at showing how to use the resources provided by a theory of “possibilia” to produce analyses of a host of modal locutions. But his realism about possible worlds consists in much more than inclusion of such entities into his ontology; indeed, it would probably be better to call Lewis a “reductionist” about modality—reductionist in a way that distinguishes him from virtually every other philosopher of modality. For a typical believer in possible worlds will, if asked to explain what they are, give an account that uses modal notions at some crucial point. Perhaps she will say that possible worlds are maximal consistent sets of sentences (in some appropriate language); or perhaps she will say that they are certain kinds of maximal properties that reality as a whole could have instantiated. Lewis says no such thing: he offers a characterization of possible worlds—and thus of modality generally—in explicitly non-modal terms. This complete subordination of the modal to the non-modal gives his philosophy of modality a quite radical character, and also sheds light on some of his seemingly independent views about the modalities involved in such concepts as causation, law of nature, and chance. (For example, Lewis rejects philosophical accounts of laws of nature that rely on any primitive modal notions.)