 In general, to evaluate a counterfactual, say “If it were the case that X1,…,Xn, then …”, one replaces the original equation for each variable Xi with a new equation stipulating its hypothetical value,while keeping the other equations unchanged; then one computes the values for the remaining variables to see whether they make the consequent true. This technique of replacing an equation with a hypothetical value set by a ’surgical intervention’ enables us to capture the notion of counterfactual dependence between variables: