 This version of the criterion has Frege’s formulation as a special case, since sentences are, of course, expressions; and it solves the problem with obvious truths, since it seems that substitution of sentences of this sort can change the truth value of a propositional attitude ascription. Furthermore, the criterion delivers the wanted result that coreferential names like “Superman” and “Clark Kent” differ in sense, since a rational, reflective agent like Lois Lane could think that  (17)  is true while withholding assent from  (18).