The reason is that "meaning exists only where there is a distinction between Intentional content and the form of its externalization and to ask for the meaning is to ask for an Intentional content that goes with the form of externalization" (ibid.
To know English is to know, for example, that the form of words 'there are cats' is standardly used to express the thought that there are cats; and that the form of words 'it's raining' is standardly used to express the thought that it's raining; and that the form of words 'it's not raining' is standardly used to express the thought that it's not raining; and so on for in(de)finitely many such cases."
Human linguistic behavior is, to be sure, more complicated than the behavior of, say, bees, but this difference seems to be quantitative, rather than qualitative. With respect to the mentalistic conception (here in the Brentanian and Searlian form of basing meaning on intention), Quine (1960, p.
However, there is no reason to abstain from making models of the semantics of language in the form of functions assigning expressions some kinds of objects10.
A predicate typically connects with a name to form a sentence, which is either true or false, depending on the name, or, more precisely, by the referent of the name.
A question of the form 'What is an F?' can be answered only by recourse to a further term: 'An F is a G.'
I have indicated why I think this conception of a theory of language is futile: I have indicated why the mentalistic conception of meaning is problematic (only hinting at all the complexities discussed at length by Wittgenstein and his direct and indirect followers - in the American context especially by Sellars, Quine and Davidson); and I have also indicated that any theory worth its name must concern itself with public universals rather than with private particulars, and must envisage an intersubjectively understandable "form" or "structure".
Realism (in the spirit of Katz and Postal), on the other hand, seems to claim not only that linguistic data, to be construable as such, must display some regularities and appear as instances of a realistic "form"; they seem to claim also that these realistic entities are accessible in a direct way. Katz and Postal write about "sentential structure" which can be examined to see if it is "at some grammatical level logically significant" (ibid.
As de Saussure puts it: 'Language is a form and not a substance' (de Saussure 1931: 122).
This means that as soon as we take compositionality as a principle, then the value issuing from the differentiation between truth and falsity is a value quite different form the truth value.