If the descriptivist theory says that the sense of a name is a description, then why do the cognitive values of the two differ? i.e. One could believe the first, but not the second.
Thanks.
There is more than one version of the descriptive theory of names, but on one common variant, names refer to their referents via their meanings. On this view, "Superman" and "the flying man with the red cape" have the same referent, but pick out that referent in different ways, because "Superman" might be associated with any number of different descriptions. Hence, it is possible for "Superman is the flying man with the red cape" to be true but not knowable a priori. If Alice has heard of Superman, but does not know the colour of his cape, she might associate the name "Superman" with a different description from someone who has seen him.
Similarly, "Hesperus is Phosophorus" is true, but not knowable a priori, on the descriptive theory, since the names "Hesperus" and "Phosphorus" have meanings that are distinctly different descriptions, while sharing the same referent.
Of course, the descriptive theory raises as many problems as it solves, since there might be no unique description associated with a given name, and names may succeed in identifying a referent without requiring a unique description. Also, as Kripke pointed out, names typically designate rigidly in modal contexts, while descriptions do not. Some names are straightforwardly descriptive, such as "Jack the Ripper", which just means "the person who committed the Whitechapel murders". But most names don't work that way.
There is more information in the SEP article on Reference.
For example, consider Kripke's modal arguments.
Precisely, modal constructions of proper names do not behave like descriptions. For Kripke, proper names are rigid designators and descriptions (in general) are not.
If 'Superman' abbreviates the description 'is the flying man with a red cape' then take the sentences:
The former is false and the latter (identity statement) is, of course, true. In a possible world, there might have been another 'flying man with a red cape' who is Uberman, instead. (There could even be a similar man in the very same world.)
Analogously, here:
Thus, the conclusion is that the proper name and the description are doing different things here, they aren't equivalent. For Kripke, rigid designators 'tag' the same thing in every world, (hence they designate "rigidly"). Descriptions, on the other hand, pick out different objects in different worlds.
Kripke attacks descriptivism in 'Naming and Necessity' at length. There are two major examples in there:
1. Feynman's speaker:
Consider Richard Feynman, to whom many of us are able to refer. He is a leading contemporary theoretical physicist. Everyone here can state the contents of one of Feynman’s theories so as to differentiate from Gell-Mann. However, the man in the street, not possessing these abilities, may still use the name “Feynman.” When asked he will say: well he’s a physicist or something. He may not think that this picks out anyone uniquely. I still think he uses the name “Feynman” as a name for Feynman.
2. Gödel's incompleteness theorem:
Suppose that Gödel was not in fact the author of this theorem. A man named ‘Schmidt’, whose body was found in Vienna under mysterious circumstances many years ago, actually did the work in question. His friend Gödel somehow got hold of the manuscript and it was thereafter attributed to Gödel. On the view in question, then, when our ordinary man uses the name ‘Gödel’, he really means to refer to Schmidt, because Schmidt is the unique person satisfying the description, ‘the man who discovered the incompleteness of arithmetic’… So, since the man who discovered the incompleteness of arithmetic is in fact Schmidt, we, when we talk about ‘Gödel’, are in fact always referring to Schmidt. But it seems to me that we are not.
To put it briefly, the cognitive values can differ because names and descriptions can name and describe nonmentally real objects or can instead name and describe mentally experienced object-conceptions. If we assume that there is only one Superman (if not, then "Superman is Superman" could be false, just as "Jupiter is Jupiter" is false if one "Jupiter" refers to the planet but the other "Jupiter" refers to the mythical Greek god), and if we assume that there is only one flying man with a red cape, then if we take the sentences as referring to nonmentally real objects, "Superman is the flying man with the red cape" is just as necessarily true as is "Superman is Superman," for the extension of "Superman" and the extension of "the flying man with the red cape" are identical. But if we take the sentences as referring to mentally experienced object-conceptions, then either may be true and either may be false (assuming the conceiver is unaware that there is only one Superman and does not intend the word "Superman" the same way in both uses), for the intensions need not be the same (but could be).
