 since it is an instance of ‘if it had been that \(A\), then it would have been that \(A\)’. Yet substituting ‘Phosphorus’ for the first occurrence of ‘Hesperus’ in the consequent of (5) gives (4). Since (5) is true and (4) is (supposedly) false, substitution of identicals has failed. This failure of substitutivity, Williamson claims, is a bad result, for counterfactuals should not create opaque contexts. 