Here's a quick argument against views according to which there are conditions external to the present individual that must be met in order for the individual's present thoughts to qualify as meaningful. (One such view is that language and thought are irreducibly social; another would be historical/'teleological' views according to which evolutionary history and natural 'function' are essential for meaningful thought.)
(1) I can know just by introspection that I have meaningful thoughts. (Cf. Descartes)
(2) If the true theory of mental representation entails that there are external conditions X on meaningful thought, then -- by inferential closure -- if I know this fact, and I know that I have meaningful thoughts, then I'm in a position to know that X obtains.
(3) I can't know just by introspection that some external condition X obtains. This remains so even if I have background knowledge (derived through a priori reflection, say) of the true theory of mental representation.
Therefore, the true theory of mental representation does not place any external conditions on meaningful thought. Isolated Swampman (an intrinsic duplicate of me, but who can only just now popped into existence, and hence my historical and social relations) has meaningful thoughts just as I do.
I'm guessing this kind of objection must have been raised before. Anyone know of a convincing response?