"For a parcel of matter to take on a form is for it to become a thing of the kind the form is a form of; for example, for it to take on the form of triangularity is just for it to become a triangle. Now for the intellect to grasp the nature of a thing is just for it to take on the form of that thing. And in that case, if the intellect were material, it would become a thing of the kind that it grasps; for instance, it would become triangular when it grasps the form of triangularity. But this is obviously absurd. So the intellect is not material."
-- Edward Feser, "Plato's affinity argument"
Recall that in De veritate q.I a.i St. Thomas says says of the soul (anima): "quae quodammodo est omnia" (it is in some manner all things).