Aquinas frames human knowledge between two other types of knowledge: between angelic cognition and the sense-cognition of non-human (sub-human) animals (and perhaps plants). Angels are incorruptible and disembodied intellects; their cognition is not the act of a bodily organ and is therefore unconnected with matter. They have a direct knowledge of immaterial forms, because they are directly illuminated by divine light. They perceive without sense-data. Angelic knowledge of matter or material objects is deduced from their knowledge of forms; Thomist forms contain within themselves enough information that every specific instantiation of them can be deduced from them; they are therefore probably somewhat richer than Plato's forms. Angels do not obtain or receive sense-data; one might therefore ask if they are blind and deaf.
By contrast, the cognition of animals, and of any other category of being which might be considered a corruptible material substantial form, is based exclusively upon sense-data. This sensory cognition is devoid of any knowledge of forms; such creatures are necessary unaware and ignorant of forms, because their cognition is purely the act of bodily organs and nothing else. When the body dies, the form dies. Animalistic cognition knows only materials particulars, of which it forms phantasms (mental images) by collecting sensible species (sense data) through its sensory organs. This is a thoroughly physical process.
The "middle" place to which Aquinas then assigns human cognition is between angelic and animalistic. The human intellect is incorruptible: Platonic, it can exist by itself without a material body. Intellectual activity is not a function of bodily organs. The cognitive power of the soul is, or relates to, the form of the body. The human intellect understands the material world through phantasms; it deduces forms via the material world. Aquinas uses designates as 'intelligible species' those forms which the mind deduces. ('Forms' and 'intelligible species' might be very nearly synonymous in Thomist vocabulary.)
We have seen how Aquinas locates human intellect in a "middle" position between angelic and animalistic cognition. He faces the challenge of showing that he can harmonize his system with both Augustine and Aristotle. The process of divine illumination is central; Augustine locates forms in God, or more precisely in the divine intellect. Augustine says that we know material objects through God's ideas. According to Augustine, God illuminates our minds with forms. By contrast, Aquinas has indicated that the human intellect deduces forms from phantasms informed by sensible species. How then can Aquinas claim to be in harmony with Augustine? Aquinas will claim that his system shows humans to be indirectly illuminated by God. Augustine seems to indicate a more direct illumination, but if Aquinas can explain some manner of indirect illumination, he can then say that he agrees with Augustine that the human intellect receives divine illumination, and downplay the distinction between his indirect illumination and Augustine's direct illumination. In any case, it will be imperative for Aquinas to avoid giving the impression that the human intellect is self-illuminating.
Turning to the issue of the human will, Aquinas is concerned to show that he is in harmony with Aristotle. Aristotle posited the will as a rational appetite which is necessarily directed toward happiness. Aquinas agrees, and one question about the human will is about the nature of the necessity which directs it toward happiness. Such a necessity could be at odds with some notions of freedom. By contrast, Aquinas and Aristotle define 'passion' as a sensory appetite. Aquinas, who is concerned to weld Aristotelian psychology to Christian ethics, posits a second function of the will: free choice. This activity of free choice is not necessary, and therefore contingent, and is directed toward practical goods. Practical goods are, or are thought to be, means to an end, where the end is happiness. This allows for the human will to be at times misguided, when we freely chose a means, but do not know that these means will fail to achieve the desired end.
The will as rational appetite, which is determined by its object, inasmuch as happiness is necessarily the goal of the will is contrasted with the will as free choice, which is not determined by its object. Even in the former case, Aquinas sees the necessity as uncoerced: happiness necessarily attracts the will, but the will is not forced toward it.
Aquinas raises some murky issues around the question of whether the notion of Providence conflicts with free will. First arises the older question, going back at least to Augustine if not earlier, about the compatibility of our free action with God's foreknowledge. How can God foreknow our free actions prior to creating anything, specifically, prior to creating our wills which will make choices freely? To the extent that God's foreknowledge is based upon His knowledge of our will, it would seem that He designed our wills, and so His foreknowledge comes into tension with the freedom of those wills.
