 Then Basic Law V says that the predicates "Fx" and "Gx" have the same extension if and only if ∀x["Fx" ↔ "Gx"].The set of Fs is the same as the set of Gs just in case every F is a G and every G is an F. (The case is special because what is here being called the extension of a predicate, or a set, is only one type of "value-range" of a function.