  The similarity analysis has come in many varieties and formulations, including the system of spheres approach informally described in §2.1. That formulation is easiest for comparison to strict analyses. But there is a different formulation that is more intuitive and better facilitates comparison among different similarity analyses. This formulation appeals to a (set) selection function f, which takes a world w, a proposition p, and returns the set of p-worlds most similar to w: \(f(w,p)\).[30] \(\phi&gt;\psi\) is then said to be true when the most f-similar \(\phi\)-worlds to w are \(\psi\)-worlds, i.e., every world in \(f(w,{\llbracket}\phi{\rrbracket}^f_v)\) is in \({\llbracket}\psi{\rrbracket}^f_v\). The basics of this approach can be summed up thus.