  simply enforces that \(f(w,p)\) is indeed a set of p-worlds. Recall that \(f(w,p)\) is supposed to be the set of most similar p-worlds to w. The other constraints correspond to certain logical validities, as detailed in the supplement Formal Constraints on Similarity. This means that Pollock (1976) endorses the weakest logic for counterfactuals and Stalnaker (1968) the strongest. It is worth seeing how, independently of constraints (b)–(d), this semantics invalidates an antecedent monotonicity pattern like Antecedent Strengthening.