Statesboro, GA, USA
No, Brad, you don't have to defend Kant.
I just digressed to a discussion of Kant, since it caught my interest a bit more than the current discussion of whether "imperative" or "obligatory" is the better word. Incidentally, I think Kant's choice of "imperative" over "obligatory" is of no real significance, other than the fact that the former seems more impersonal, and thus rings better with his non-authoritarian view of ethics. Otherwise the words are pretty much synonymous.
I guess it just seems to me that (whether you agree with Kant or not) a discussion of his views on ethics may cut to the underlying issues of this thread, more directly. Because whether or not morals come from "reason" or not, the anti-rational philosophers (Shopenhaur comes to mind) at least taught us to question what authority lies in reason. What makes any of the dictates of reason truly "imperative"?
As far as your example of having a drink ... I don't think that actions or urges would necessarily correspond to reason, based upon the categories of "internal" and "external". The urge to drink is itself an internal desire. One may disregard what people think, in order to make the right choice. But one may also disregard what people think, in order to make the wrong choice. ... "I'm going to have another drink, I don't care what people think."
My understand of Kant's use of "heteronomic" has to do with external authority ... such as moral teachers, religious institutions, God as orthodox Christian Theism holds, etc ... Man becomes autonomous, in the sense that he must "by his own reason" make moral decisions. Yet Kant crosses his own Phenomenal / Noumenal divide, when he speaks of reason being universal. Maybe he escapes that charge when he says "as if" ... but then it begs the question, why "as if universal" , if it's only "as if"?