 So the contrapositive asserts that if \(\epsilon F \neq \epsilon G\), then \(\neg \forall x(Fx \equiv Gx)\).  But in the case where the material equivalence of \(F\) and \(G\) is a necessary condition for \(F = G\), i.e., in the case where \(\neg \forall x(Fx \equiv Gx)\) implies \(F \neq G\), then Va implies that if \(\epsilon F \neq \epsilon G\), then \(F \neq G\), i.e., that whenever the extensions of \(F\) and \(G\) differ, the concepts with which they are correlated, namely \(F\) and \(G\), differ. This means that the correlation between concepts and extensions that Basic Law V sets up must be a function – no concept gets correlated with two distinct extensions (though for all Va tells us, distinct concepts might get correlated with the same extension). Frege noted (in the Appendix to Gg II) that this direction of Basic Law V doesn’t seem problematic. 