 But his largest contributions were in metaphysics. One branch of his metaphysics was his Hume-inspired reductionism about the nomological. He developed a position he called “Humean supervenience”, the theory that said that there was nothing to reality except the spatio-temporal distribution of local natural properties. And he did this by showing in detail how  laws, chances,  counterfactual dependence,  causation,  dispositions and colours could be located within this Humean mosaic. The other branch of his metaphysics was his  modal realism.  Lewis held that the best theory of modality posited concrete possible worlds. A proposition is possible if and only if it is true at one of these worlds. Lewis defended this view in his most significant book, On the Plurality of Worlds. Alongside this, Lewis developed a new account of how to think about modal properties of individuals, namely  counterpart theory,  and showed how this theory resolved several long-standing puzzles about modal properties.