  It is worth noting the idea of another sense of belief-ascription which would not be vulnerable even prima facie problems of extensionality: so-called de re belief, as distinct from de dicto belief. In the 1950s Quine argued that there must be such a sense. In the late 1960s, however, he abandoned the idea, for lack of clear standards of when it is correct to ascribe a de re belief to someone. (See Quine, 1956 for a statement of the distinction, and Quine, 1973 for retraction.) Other philosophers, however, continue to hold the idea. (See, for example, Kaplan 1968, Kripke 1979.)