When Thomists and other scholastics say that the proper object of the human mind is being concretized in the sensible, or the illuminated/abstracted phantasm, “proper object of the human mind” means “an object peculiar to the human mind”. The claim is that neither apes nor angels have ever seen being concretized in the sensible, or, if you like, the only time an angel has ever seen such a thing is when he was considering the immanent action of a human being. His awareness of such an object is entirely mediated by a consideration of the human mind – there is no immediate awareness of the object we spontaneously and immediately perceive. Even a divine nature has no immediate awareness of an illuminated sensible object, because apart from a human mind there is no such thing.
Notice that proper object is not the same thing as an object one is restricted to, or, even if one does take it this way, “proper object” is taken as located between limits of pure intellection and pure sensibility, and so one who speaks of a proper object is also able to speak of points of contact with higher and lower sorts of knowledge. The contours of the proper object can only be given by the negative space of pure intellection and pure sensibility. While it’s clear to us that we can only understand an angelic object by negation or eminence, it is also true that to understand the conscious objects of non-human animals also requires negation and the emptying of something eminent from its object. Animals might, for all I know, see “brown”, and an analysis of some of their eye structures would seem to support this, but there is no evidence for (and nothing but evidence against) that they have ever seen a brown thing, or the thing called brown. Human beings all see some supporting reality “behind” or “underneath” brown, whether it is an Aristotelian or Nominalist substance, a Platonic idea, a Berkelian or Idealist mind, a Pythagorean/ Galilean geometry-number, a positivist fact, etc. (The Buddhist sunyata or sense that “nothingness” is what is behind all things is still at least logically the same as those ideas that put positive reality behind the sensible. Does anyone think that rats, or even apes, could see sunyata?) Even if one defined a thing in a functionalist or pragmatist manner, to be either of these is to take some fact about things on principle, and mere functional action is not the same thing as being functionalist on principle.
The world of sensible things is something only we can experience from within. We are in continual danger of collapsing the “sensible” into “thing” (that is, into the reality “behind” or “underneath” it, a reality that includes even “emptiness”) and the thing into the sensible.