Frege's puzzles

Frege's puzzles are puzzles about the semantics of proper names, although related puzzles also arise in the case of indexicals. Gottlob Frege (1848–1925) introduced the puzzle at the beginning of his article "Über Sinn und Bedeutung" ("On Sense and Reference") in 1892 in one of the most influential articles in analytic philosophy and philosophy of language. The term "Frege's puzzle" is commonly applied to two related problems. One is a problem about identity statements that Frege raised at the beginning of "On Sense and Reference", and another concerns propositional attitude reports. The first problem considers the following sentences: Hesperus is Hesperus. Hesperus is Phosphorus (Lucifer). Each of these sentences is true, since 'Hesperus' refers to the same object as 'Phosphorus' (the planet Venus which few months apart can be seen as the brightest star of morning or of evening). Nonetheless, (1) and (2) seem to differ in their meaning or what Frege called "cognitive value". (1) is just a truth of logic that can be known a priori, whereas (2) records an empirical truth that was discovered by astronomers. The problem, however, is that proper names are often taken to have no meaning beyond their reference (a view often associated with John Stuart Mill). But this seems to imply that the two statements mean the same thing, or have the same cognitive value. Frege proposed to resolve this puzzle by postulating a second level of meaning besides reference in the form of what he called sense: a difference in the mode of presentation or the way an object can be "given" to the observer. Thus 'Hesperus' and 'Phosphorus' have the same reference, but differ in sense because they present Venus in different ways. The second puzzle concerns propositional attitude reports, such as belief reports. Ordinarily, coreferring names are substitutable salva veritate, that is, without change in truth value. For example, if 'Hesperus is bright' is true then 'Phosphorus is bright' is also true given that 'Hesperus' and 'Phosphorus' refer to the same planet.