 In “Counterfactual Dependence and Time’s Arrow” (1979c), Lewis responded by saying more about the notion of similarity. In particular, he offered an algorithm for determining similarity in standard contexts. He still held that the particular measure of similarity in use on an occasion is context-sensitive, so there is no one true measure of similarity. Nevertheless there is, he thought, a default measure that we use unless there is a reason to avoid it. Here is how Lewis expressed this default measure.