The first dear and consistent version of cr can be found in Gottlob Frege's paper 'On Sense and Reference' of 1892. Of course, Frege's theory is not about mental reference as such but about linguistic reference: he does not speak of acts, contents and objects but of expressions, senses and referents. (We use 'referent' instead of 'reference' because the latter seems to designate the act of referring, the former the entity referred to.) However, there is a direct correspondence between the mental and the linguistic terminologies, and the two theories can in fact be translated, one into the other:
Frege Expression Sense Referent
Content theory Act Content Object
According to Frege it is the problem of identity that makes it necessary to distinguish between the sense and the referent of a linguistic expression. If we say that the morning star is the same as the evening star, we do not mean that there are two entities that happen to be one entity; rather, we mean that there is one entity to which we refer by using two different expressions. Identity, in other words, is a relation not between two entities but between two names.