Aquinas puts forward two different, and conflicting, interpretations of Themistius’s account of the intellect. In his earlier interpretation of Themistius, Aquinas understands him to hold the position that both the possible and agent intellect are separate and incorruptible, existing apart from individual human souls but shared in by individual souls in the process of knowing. In De unitate intellectus contra averroistas, however, Aquinas radically departs from this reading, hailing Themistius as a genuine interpreter of the Peripatetic position, while decrying Averroes’s perversion of both Themistius and Aristotle. This paper examines these competing interpretations of Themistius’s account of the intellect in his Commentary on the De anima of Aristotle, focusing on two issues central to its interpretation: (1) the nature of intellect insofar as it is separate, impassive, and unmixed, and (2) whether the productive intellect is one or many.

In his philosophical works, Aquinas spends some effort establishing why cognitive beings differ from those that are not able to have a cognitive, i.e., intentional, grasp of the exterior world. Prima facie, the matter is clear, since only those beings acquire knowledge that have the proper powers to do so. One remark, however, while discussing the nature of change in the process of visual perception, strikes the reader as particularly odd, since Aquinas states that “a ‘spiritual alteration’ occurs in virtue of a species’ being received in a sense organ or in the medium in the manner of an intention.” Whereas it is not problematic to think that perceptions and thoughts are intentional, it seems peculiar to talk of the species in the medium as being received “in the manner of an intention.” While current interpretations propose that Aquinas’s account is either erroneous or in need of rectification, I would like to argue that the notion of mind-independent or non-cognitive intentions, which follows the Avicennian tradition, is rooted in a peculiar theory of sensible form. Given that the intentions in the medium make sense, it is, however, important to show that they differ from those intentions that are apprehended by cognitive powers. For this purpose, I will try to trace the underlying physics for cognitive change, showing that an account in terms of qualitative change leads one to posit a proper recipient of sensible forms, i.e., the sense powers.