(CG 2/48). That Subsistent Intelligences have Free Will (Book 2: God The Origin of Creatures) (Summa Contra Gentiles) (Aquinas, Thomas, St.)

IF the understanding were a corporeal substance, intelligible ideas of things would be received in it only as representing individual things. At that rate, the understanding would have no conception of the universal, but only of the particular, which is manifestly false.

4. If the understanding were a corporeal substance, its action would not transcend the order of corporeal things, and therefore it would understand nothing but corporeal things, which is manifestly false, for we do understand many things that are not corporeal.

5. There can be no infinite power in any finite body: but the power of the understanding is in a manner infinite in the exercise of intelligence: for it knows the universal, which is virtually infinite in its logical extension.

7 and 8. Of no bodily substance is the action turned back upon the agent. But the understanding in its action does reflect and turn round upon itself: for as it understands an object, so also it understands that it does understand, and so endlessly. Hence Holy Scripture calls intelligent subsistent beings by the name of 'spirits,' using of them the style which it is wont to use for the incorporeal Deity, according to the text, God is a Spirit (John iv, 24). Hereby is excluded the error of the ancient natural philosophers, who admitted no substance but corporeal substance: which opinion some have endeavoured to foist into the Christian faith, saying that the soul is an effigy of the body, a sort of outline contour of the human body.