While it might seem intelligible to wonder about the foundations of any knowledge that we have, and to seek arguments for its reliability, it is a serious mistake to seek an argument for the possibility of intelligible conversation. If argument is possible, then so is intelligible conversation.

This doesn't mean that questions such as 'how do we speak?' and 'how can we speak?' don't make sense in certain contexts - e.g. the neurology of language, linguistic theory etc.. These disciplines can give us insights into the mechanical or computational processes involved in speech, or into linguistic structure. They may even show us how certain structures or processes are necessary for reliable communication, in the sense that (under interpretation) all languages incorporate these structures and processes.

What they cannot show is that some set of mechanical, cognitive, or structural criteria guarantee intelligibility - are not just necessary, but also sufficient. This is a corollary of Davidson's observations on interpretation - that there is always more than one solution to the translation problem, because we must make assumptions about the normative, epistemological, and motivational character of any potential interlocutor. The 'Principle of Charity' renders the interoluctor 'human', but not only is this a normative posture adopted by the translator, there are also quite likely ot be different renderings of this humanity.

We cannot get from grunts to sense without making normative choices, however 'obvious' these choices might seem to a routine user of a human language.

And yet sense is guaranteed, within any conversation, by the possibility of that conversation. 'This conversation is impossible' is not a possible conversational move.

Neurological, cognitive, and linguistic theories - or, indeed, epistemological theories - are all parts of some intelligible conversation or another. They cannot provide a better demonstration of the intelligibility of that conversation than their own intelligibility already guarantees.

To ask a question about that intelligibility is to ask a question about the possibility of asking questions.