 On a dual view, worlds are taken to be sets of facts, a world being identified with its own domain: for every world \(w, F_w = w\). On that view, No Twins is trivially true. Variants on that view take worlds to be pluralities, or mereological sums of facts. Cresswell (Cresswell 1972) is in the set camp, Wittgenstein (TLP), it seems, in the plurality camp.