The propositional attitudes
DOI link for The propositional attitudes
The propositional attitudes book
Gottlob Frege held that something very similar goes for indirect quotation, and for the related expressions of propositional attitude such as belief. Nathan Salmon published his Frege's Puzzle in 1986; it was one of several attempts to combine an approach to the propositional attitudes with direct reference; others have included Mark Richard and Scott Soames. Nathan Salmon allows the free substitution of co-referring proper names within the scope of operators of propositional attitudes. Frege's explanation is that that Venus is a planet is not a sentence, but a complex singular term denoting a proposition, the sense of 'Venus is a planet'. The word 'that' converts a sentence into a singular term referring to the sense of the attached sentence. For Frege, in hyper-intensional contexts terms undergo a referential shift. And the shift can happen multiple times, as in Dudley believe that Adam believes that the Evening Star is a planet.