 Frege’s claim that propositional attitude verbs induce a reference shift allows him to preserve a substitution principle. The fact that (1) is true while (4) is false does not show that substituting co-referring singular terms within the scope of attitude verbs is illegitimate; instead, for Frege, it shows that ‘Superman’ and ‘Clark’, as they occur in (1) and (4), are not co-referring.