Thomas Aquinas, in great measure, although not slavishly, following Avicenna, organizes our sensory experience into particular sensory powers. The external powers are the senses in our usual sense; vision, hearing, touch, taste, smell. The internal powers or senses process these: the common sense is, so to speak our sense of sensing and thus is how we are aware that we are sensing in such-and-such way and also is our sense of our sensations relating to each other (our sense of co-sensing, e.g., that the sound we hear is co-sensed with the sight of something rushing by); the imagination retains and recombines what we have sensed; the estimative is concerned with 'intentions', i.e., principles organizing our senses in particular ways, and in brute animals, for instance, is their sense of safety and danger, or of things as attractive or repulsive; and the memorative retains these intentions, and is therefore among other things the sense that something has been retained by the imagination. In human beings, however, Aquinas, we have instead of an estimative sense a cogitative sense, which does still seems to have, as part of its lower act some estimative role like that found in other animals, but whose higher and principal act in human beings is ministry to the intellect.
As Aquinas puts it, the excellence of the cogitative sense compared to the estimative sense in other animals lies in its "affinity and proximity to universal reason, which, in a sense, overflows into them" (ST 1.78.4 ad 5). The formality under which the cogitative sense handles our sensory experience is cognition of individuals as under a common nature. Unlike the intellect, it has no conception of universals as such, but it does identify and create particular patterns and groupings and recognize individual, singular, particular things as members of those groupings and components of those patterns. Because it serves as an instrument for intellectual acts, the cogitative sense is also called the passive intellect and the particular reason. It is not strictly intellect, at all, but it organizes and disposes our sensory experience in ways that facilitate or are responsive to things like intellectual abstraction. It is likewise not strictly reason at all, but it re-organizes our internal sensory processing in light of reasoning, and most importantly is how we think about singular, particular things. The latter means that it has a central role in practical problem-solving and moral reasoning; prudence as a moral virtue primarily works by organizing the cogitative sense.
I have been thinking about the cogitative sense because I am re-reading Kant's Critique of Pure Reason, and it suddenly struck me that almost everything Kant describes in the work is concerned with what Aquinas would call the cogitative power, and if you understand it in this way as Kant's 'reason' actually being the cogitative sense, suddenly so many things about Kant's argument start to make sense.
This is not an accident, but arises from the structure of the problem that Kant has set himself. Kant concedes a lot to empiricism, and one of things he concedes is that we only have empirical objects of cognition. All objects of cognition must come from sensation, imagination, or empirical apperception (sense of self). Thus all of our cognition of objects consists of acts that are what Aquinas would call acts of the internal senses. However, Kant is not an empiricist; he (correctly) recognizes that empiricism cannot possibly account for our actual thought and experience. Attempting to work out the possible conditions for our actual thought and experience, he sets himself the problem of how our sensations, imaginations, and empirical apperceptions can possibly have the unities they actually do; he calls this synthesis, and, using transcendental arguments, he establishes that these syntheses must conform to concepts and possible judgments, and that to make sense of any such synthesis and its conformity, you must take the concepts and the forms of judgments to be a priori, and to make sense of our actual experience of objects, you must be able to make the distinction between the phenomenal (the empirical) and the noumenal; however, given the empiricist restriction, we cannot take the noumenal as object, so it can only be a concept, held a priori, that serves as a limit-concept for the phenomenal, without which the latter cannot be interpreted as we inevitably interpret it.
Now, the highest cognitive power that specifically concerns empirical objects in Aquinas's scholastic account is the cogitative sense. Thus Kant's empiricist restriction of objects to the empirical means that his analysis of human thought and experience cannot rise higher than the cogitative power. The cogitative power is what synthesizes everything done by all of our other senses, internal or external, and it is also apperceptive, in the sense that in at least a rudimentary way it is a sense of self -- it is how we cognize ourselves as individuals in relation to other individuals, and are able to compare and contrast ourselves with other things, because it is how we cognize anything as individual. However he (correctly) recognizes that cogitation works the way it does only because of what it presupposes, which makes its particular syntheses possible and which can be dimly recognized as the limits or boundaries in light of which everything else is organized. Everything that Kant calls 'transcendental' or 'a priori' or 'noumenal' is what in scholastic terms would be called (depending on the case) intellectual acts and concepts or intelligible objects. In St. Thomas's terms, Kant, starting with the cognitative sense, by transcendental argument establishes that its actions of synthesis require the intellect as a condition for their possibility. However, since he does not rise above what can be found in the cogitative power, he can only consider the intellect 'remotively' -- going simply on what the cogitative power provides, we cannot know what the intellect is or how it works, but only that it is and that our experience is organized in light of it and what it is not. This apophatic character is why Kant has so much difficulty in characterizing the noumenal.
As one might expect from how I have described this, I think Aquinas at the fundamental level has the stronger position. Kant concedes too much to the empiricists, and many of the weirder aspects of his epistemology and critique arise directly from those concessions. At the same time, he shows that the empiricists can't be right, even given those concessions, because human experience does not work they way they claim it should and their very limited principles can't explain what we actually experience. In doing this, he's not really doing anything that any other rationalist would, although he does it very well, particularly given how much he has conceded. But what he ends up establishing is that even with those concessions you keep running up against something that goes beyond what those concessions can directly allow, much less explain. We have concepts and principles (or rules, as he often calls them) that, with respect to the kinds of thinking he has been considering, are 'transcendental', and understanding the phenomenal content of experience requires positing the noumenal as a limit. Yet the noumenal has to be more than just a limit, although Kant due to the empiricist concessions is unable to say anything, or at least anything very consistent, about it. All of this can be cleared up simply by recognizing that we have, in however limited a form, a higher cognitive ability than Kant allows when he makes his empiricist concessions. That ability, all call 'intellect'.
