 This was, in many ways, an attractive development of Frege’s vision. But it also raised a skeptical worry. Perhaps the structural mismatches between sentences of a natural language and sentences of a Fregean Begriffsschrift are so severe that one cannot formulate general rules for associating the sentences we ordinarily use with propositions. Later theorists would combine this view with the idea that propositions are sentences of a  mental language  that is relevantly like Frege’s invented language and relevantly unlike the spoken languages humans use to communicate; see Fodor (1975, 1978). But given the rise of  behaviorism,  both in philosophy and psychology, this variant on a medieval idea was initially ignored or ridiculed. (And it does face difficulties; see  section eight.)