 Lewis’s alternative theory relied on changing the definition of counterfactual dependence. The theory in “Causation” was based on what he came to call ‘whether-whether’ dependence. What’s crucial is that whether B happens depends counterfactually on whether A happens. The new theory was based on what we might call ‘how-how’ dependence. Lewis says that B depends on A if there are large families of counterfactuals of the form If A had happened in this way, then B would have happened in that way, and the ways in which B would happen are systematically dependent on the ways in which A happens. How much A influences B depends on how big this family is, how much variation there is in the way B changes, and how systematic the influence of A on B is. He then defines causation as the ancestral of this notion of counterfactual dependence.