 And so on. It is this kind of question—albeit not always phrased in this way, and accompanied by definite views about what constitutes a philosophically appropriate answer—that animate what we might call Lewis’s “applied metaphysics”: the application of his basic positions in ontology and modality to a range of perennial metaphysical topics. Note that the reductionist character of his approach comes out when we pursue the obvious follow-up questions: For example, what is it for one event to counterfactually depend on another? Roughly, it is for the closest possible world in which the second does not occur to be a world in which the first does not occur. What is it for one world to be closer to actuality than another? We’ll skip the answer for now—but rest assured that it and the answers to subsequent follow-up questions are designed to hang together in such a way as to collectively display how facts about what causes what ultimately reduce to facts about fundamental ontology. And so it goes, for personal identity, free will, the mind, knowledge, ethics, laws of nature, you name it.