 Since truth-value is on the level of reference, and the corresponding words of (2a) and (2c) all have the same referents, the resulting truth-values for (2a) and (2c) will be the same; but they are supposed to be different. So Frege makes the ingenious suggestion that it is an effect of embedding in intensional contexts (he only considered clausal verbs) that expressions in such contexts no longer denote their customary references, but rather their customary senses. Using ‘iff’ henceforth to abbreviate ‘if and only if’, then (1) intended as (2a) is true iff the reference of ‘Lex’ stands in the fearing-true relation to the switched reference of ‘Superman is nearby’, namely, its customary sense. Now we have our explanation of why merely interchanging customarily co-referential expressions in (1) can produce falsehood from a truth: the substitution does not preserve reference, since the names are now denoting their customary senses. However, if (1) is intended as (2b), there will be no truth-value switch, since there is no name-reference switch: in (2b) ‘Superman’ is not in the scope of ‘fears’, therefore it denotes its customary reference, and interchange with any other expression denoting that same referent must of necessity be truth-value preserving. (Readers in search of more detailed discussion of Frege’s notion of sense might consult the section on  Frege’s philosophy of language  in the entry on Frege. See also the entry on  propositional attitude reports,  and for other uses of Frege-style “switcher semantics”, Gluer and Pagin 2012.)