On the Immortality of the Soul

...deceived. [The world is not] eternal, but the world was founded with a beginning of time, as both others and we ourselves have discussed elsewhere. Nor did a certain illustrious Arab think it absurd—which was also related by Saint Thomas in the Summa Theologiae—that things separated from matter, which neither depend on one another nor are subject to place or time, can be infinite.

As for what pertains to the part, it is manifold. For there is a part which is perfected and perishes, and there is a part which perfects and survives living. This is indeed considered insofar as it is the soul—that is, the form and life of the body—and insofar as it is a certain essence or intelligible substance by which the body is quickened. And it is placed in that rank of things, or if you prefer, of beings, such that no other created thing, which is not the human soul, can be altogether compared and equated with it; whose existence some consider distinct from its essence or nature, and they weigh its very being both in the body and outside the body, and they maintain that both this and that are natural to the soul, although some prefer the latter rather than the former to be given the name of "nature." Others think differently, and judge that the one should be attributed to it as to a soul, and the other more congruously as to a substance; and for that reason, in dissolution itself, the whole being that belongs to man will not be abolished but will remain in the surviving soul, and by right and merit it will be held in the number of immaterial forms, fit rather to draw to itself the body to which it imparts life, than to be drawn by a body that is by its own nature transitory.

Wherefore, of itself and not by participation, it is both entirely disjoined from matter and immortal. Aristotle teaches this at great length when discussing the generation of animals: that it does not depend on the body as far as its origin pertains, but that it "enters from without" (θύραθεν ἐπισιέναι)—I speak of the rational part, which he calls nous, and which the Latins translate as mind or intellect. Aristotle frequently brings forward this same word nous, and lest some should falsely wish it to be understood as an intellect entirely disjoined from the body, the author declares himself: who, in the third book On the Soul, while he discloses its essence, teaches that he will dispute "concerning a part" (περὶ μορίου)—that is, concerning a small portion of the soul—by which it understands and becomes wise, so that he might demonstrate that he is discussing the conjoined and intimate part of the human soul, not the disjoined and separate. This same nous is placed within man in the tenth book of the Nicomacheans and is asserted to be theion, that is, divine.