The object of intellect is the quiddity or common nature of physical objects. Despite the metaphysical difficulties, Aquinas must locate these quiddities within particular material substances (§10.1). The intellect grasps the essential features of substances through information acquired from the senses, but Aquinas's empiricism is tempered by his appeal to a form of divine illumination (§10.2). Although the possible intellect starts out as a blank slate, the agent intellect reveals the natures of things through abstraction. On inspection, the role of agent intellect turns out to be obscure, and abstraction rather mysterious (§10.3). But a clear understanding of the different forms of universality shows how Aquinas's account is genuinely explanatory (§10.4). The main part of understanding takes place not in the agent intellect but in the possible intellect, through the long, hard process of reasoning (§10.5).

Quiddities

Aquinas's methodology – familiar by now – is to work from objects to actions, and from actions to capacities:

It is high time, then, to say something about the object of intellect. Surprisingly, the Treatise does not explicitly address this issue.