 On the similarity analysis, \(\phi &gt;\psi\) is true in \(w_0\), roughly, just in case all the \(\phi\)-worlds most similar to \(w_0\) are \(\psi\)-worlds. To model this notion of similarity, one needs more than a simple accessibility sphere. One way to capture it is with with a nested system of spheres \(\mathcal{R}\) around a possible world \(w_0\) (D. Lewis 1973b: §1.3)—this is just a particular kind of set of accessibility spheres. As one goes out in the system, one gets to less and less similar worlds. This analysis can be depicted as in  Figure 5.[21]