Questions Pertaining to Aquinas and Hylomorphism (the Soul and Its Intellect)

How do we know that physical organs are only capable of apprehending concrete particulars? What incontrovertible proof do we have that intellects (of the Thomistic caliber) even obtain? I admit that an intellect qua a power of the soul is logically possible (i.e., such an idea is not self-contradictory). However, I am not convinced that such a faculty is causally possible. So I guess my first line of attack--besides consulting scripture--would be to question the existence of the intellect, in the relevant sense being discussed. Secondly, I would argue that what has been called "intellect" is possibly nothing more than a higher-order process of the brain: intellection is a biological phenomenon. The brain consequently makes it possible for us to have the facility to grasp what appear to be abstract universals. Similar proposals have been made by John Searle and Nancey Murphy.