Aristotle and Aquinas on Proving the Intellect’s Immateriality

Saint Thomas Aquinas often argues for the immateriality of the intellect and often employs one argument in particular, which he also attributes to Aristotle. As he explains in his Commentary on the De Anima, "Anything that is in potency with respect to an object, and able to receive it into itself, is, as such, without that object; thus the pupil of the eye, being potential to colors and able to receive them, is itself colorless. ... Since then (the intellect) naturally understands all bodily things, it must be lacking in every bodily nature; just as the sense of sight, being able to know color, lacks all color" (Bk. III, Lect. 7, no. 680). The intellect has no bodily nature since it understands all bodily natures, and it is a condition of knowing a power that it lacks the nature it knows. Thomas Russman, however, in A Prospectus for the Triumph of Realism, argues that what we know about the nature of perception invalidates the assumptions of Aristotle and Aquinas, and because this argument for the immateriality of the intellect depends on an analogy with sensation, its conclusion is undermined. Russman reasons that just as a pink retina can receive the form of green without any hindrance, so a material intellect can receive the forms of all material things without any hindrance. Since Aristotle and Aquinas turned out to be wrong about their understanding of the senses and used this mistake as a basis for their reasoning about the intellect, there is no reason to accept their conclusion that the intellect is immaterial. Although Aquinas and Russman attribute this argument to Aristotle, the version found in De Anima 3. 4 differs from it in one crucial respect. Whereas Thomas claims that the intellect is immaterial because it knows all bodies, Aristotle says that mind is 'unmixed' because it thinks all things (panta noei).(429a18-20) This difference, I contend, makes the argument as Aristotle actually formulated it immune from the criticism of Russman, for Aristotle can draw on his own distinction between being subject to a material alteration (taking on a form in a material way) and engaging in the activity of knowing (taking on the form of an object without matter by becoming one with it).