 In order to accommodate chancy causation, Lewis (1986c) defines a more general notion of causal dependence in terms of chancy counterfactuals. These counterfactuals are of the form “If A were the case Pr (C) would be x”, where the counterfactual is an ordinary would-counterfactual, interpreted according to the semantics above, and the Pr operator is a probability operator with narrow scope confined to the consequent of the counterfactual. Lewis interprets the probabilities involved as temporally indexed single-case chances. (See his (1980) for the theory of single-case chance.)