Good question! I agree that it is very clear that loading up a body with alcohol leads to attenuation of rational abilities. So does, for example, starving, chronic pain, lack of certain vitamins, dehydration, and extreme loss of salts.

My opinion is that there is a gradient of ability to remain rational independently of body mechanics or loss thereof, and some are better at it than others. I think the appropriate answer to your question is "SOme of each and both, depending on the degree of interdependency and habituation involved."

If you inject one dose of of about 60% H2O plus 40% C2H5OH (plus traces of phenolic compounds) into your cellphone while listening to an interesting person tell a long story, the interesting story will get badly chopped up, unintelligible or even stop being heard altogether.   The question of where the dysfunction is located will certainly arise at once, no? ;>) But I appreciate this is a very over-simplified metaphor.