Then, according to what appears from the words set down here, the author introduces another proof to show that an intelligence is not a body, because its substance as well as its activity is indivisible, and each has the unity of indivisibility, which cannot be in bodies. For a body is divided in its substance by the division of magnitude and is divided in its activity by the division of time, neither of which is proper to an intelligence. But Proclus presents this in his book to prove another point, i.e., to show that an intelligence is not divided according to motion, for he says the following: "Furthermore, the identity of its activity with its substance shows that an intellect is eternal." There is force to this proof because that thing whose activity comes to it accidentally receives variation according to that activity, so that sometimes it acts and sometimes it does not act, or soemtimes it acts more and sometimes it acts less. But the thing whose activity belongs to it according to its essence acts without variation. Such is an intelligence, to which an intellectual activity belongs according ot the nature of its essence.
Thomas Aquinas, Commentary on the Book of Causes, Guagliardo, tr. (CUA Press, 1996) pp. 58-59.
One of the things I've recently been doing (off and on around other projects) is tracing some of the history of what Kant calls the Achilles of rationalist arguments on the soul. There's a family of arguments along the lines laid down by these two examples, arguing from the simplicity of the soul to its distinction from body, its immortality, etc. While Aquinas would have no problem with saying that the soul is simple in some sense (he wouldn't think it the strict and proper sense but a relative or comparative sense), he does not typically make or express approval of such arguments, as he does here. But, of course, he isn't talking about souls here, either, but pure intelligences. Precisely what rationalism does is speak of the soul as if it were a pure intelligence -- the old charge against Descartes and company that they confuse human beings with a sort of minimal angel.
