 David Lewis’s second book was Counterfactuals (1973a). Counterfactual conditionals were important to Lewis for several reasons. Most obviously, they are a distinctive part of natural language and it is philosophically interesting to figure out how they work. But counterfactuals would play a large role in Lewis’s metaphysics. Many of Lewis’s attempted reductions of nomic or mental concepts would be either directly in terms of counterfactuals, or in terms of concepts (such as causation) that he in turn defined in terms of counterfactuals. And the analysis of counterfactuals, which uses possible worlds, would in turn provide motivation for believing in possible worlds. We will look at these two metaphysical motivations in more detail in section 4, where we discuss the relationship between counterfactuals and laws, causation and other high-level concepts, and in section 5, where we discuss the motivations for Lewis’s modal metaphysics.