  As noted, this formulation makes the limit assumption: \(\phi\)-worlds do not get indefinitely more and more similar to w. While D. Lewis (1973b) rejected this assumption, adopting it will serve exposition. It is discussed at length in the supplement Formal Constraints on Similarity. The logic of counterfactuals generated by a similarity analysis will depend on the constraints imposed on f. Different theorists have defended different constraints. Table 1 lists them, where \(p,q\subseteq W\) and \(w\in W\):