Himself through Himself.
Reply to Objection 1: Return to its own essence means only that a thing subsists in itself. Inasmuch as the form perfects the matter by giving it existence, it is in a certain way diffused in it; and it returns to itself inasmuch as it has existence in itself. Therefore those cognitive faculties which are not subsisting, but are the acts of organs, do not know themselves, as in the case of each of the senses; whereas those cognitive faculties which are subsisting, know themselves; hence it is said in De Causis that, "whoever knows his essence returns to it." Now it supremely belongs to God to be self-subsisting. Hence according to this mode of speaking, He supremely returns to His own essence, and knows Himself.
Reply to Objection 2: Movement and passion are taken equivocally, according as to understand is described as a kind of movement or passion, as stated in De Anima iii. For to understand is not a movement that is an act of something imperfect passing from one to another, but it is an act, existing in the agent itself, of something perfect. Likewise that the intellect is perfected by the intelligible object, i.e. is assimilated to it, this belongs to an intellect which is sometimes in potentiality; because the fact of its being in a state of potentiality makes it differ from the intelligible object and assimilates it thereto through the intelligible species, which is the likeness of the thing understood, and makes it to be perfected thereby, as potentiality is perfected by act. On the other hand, the divine intellect, which is no way in potentiality, is not perfected by the intelligible object, nor is it assimilated thereto, but is its own perfection, and its own intelligible object.
Reply to Objection 3: Existence in nature does not belong to primary matter, which is a potentiality, unless it is reduced to act by a form. Now our passive intellect has the same relation to intelligible objects as primary matter has to natural things; for it is in potentiality as regards intelligible objects, just as primary matter is to natural things. Hence our passive intellect can be exercised concerning intelligible objects only so far as it is perfected by the intelligible species of something; and in that way it understands itself by an intelligible species, as it understands other things: for it is manifest that by knowing the intelligible object it understands also its own act of understanding, and by this act knows the intellectual faculty. But God is a pure act in the order of existence, as also in the order of intelligible objects; therefore He understands Himself through Himself.
Whether God comprehends Himself?
Objection 1: It seems that God does not comprehend Himself. For Augustine says (Octog. Tri. Quaest. xv), that "whatever comprehends itself is finite as regards itself." But God is in all ways infinite. Therefore He does not comprehend Himself.
Objection 2: If it is said that God is infinite to us, and finite to Himself, it can be urged to the contrary, that everything in God is truer than it is in us. If therefore God is finite to Himself, but infinite to us, then God is more truly finite than infinite; which is against what was laid down above ( Question [7], Article [1]). Therefore God does not comprehend Himself.
On the contrary, Augustine says (Octog. Tri. Quaest. xv), that "Everything that understands itself, comprehends itself." But God understands Himself. Therefore He comprehends Himself.
I answer that, God perfectly comprehends Himself, as can be thus proved. A thing is said to be comprehended when the end of the knowledge of it is attained, and this is accomplished when it is known as perfectly as it is knowable; as, for instance, a demonstrable proposition is comprehended when known by demonstration, not, however, when it is known by some probable reason. Now it is manifest that God knows Himself as perfectly as He is perfectly knowable. For everything is knowable according to the mode of its own actuality; since a thing is not known according as it is in potentiality, but in so far as it is in actuality, as said in Metaph. ix. Now the power of God in knowing is as great as His actuality in existing; because it is from the fact that He is in act and free from all matter and potentiality, that God is cognitive, as shown above ( Articles [1],2). Whence it is manifest that He knows Himself as much as He is knowable; and for that reason He perfectly comprehends Himself.
Reply to Objection 1: The strict meaning of "comprehension" signifies that one thing holds and includes another; and in this sense everything comprehended is finite, as also is everything included in another. But God is not said to be comprehended by Himself in this sense, as if His intellect were a faculty apart from Himself, and as if it held and included Himself; for these modes of speaking are to be taken by way of negation. But as God is said to be in Himself, forasmuch as He is not contained by anything outside of Himself; so He is said to be comprehended by Himself, forasmuch as nothing in Himself is hidden from Himself. For Augustine says (De Vid. Deum. ep. cxii), "The whole is comprehended when seen, if it is seen in such a way that nothing of it is hidden from the seer."
Reply to Objection 2: When it is said, "God is finite to Himself," this is to be understood according to a certain similitude of proportion, because He has the same relation in not exceeding His intellect, as anything finite has in not exceeding finite intellect. But God is not to be called finite to Himself in this sense, as if He understood Himself to be something finite.
Whether the act of God's intellect is His substance?
Objection 1: It seems that the act of God's intellect is not His substance. For to understand is an operation. But an operation signifies something proceeding from the operator. Therefore the act of God's intellect is not His substance.
Objection 2: Further, to understand one's act of understanding, is to understand something that is neither great nor chiefly understood, and but secondary and accessory. If therefore God be his own act of understanding, His act of understanding will be as when we understand our act of understanding: and thus God's act of understanding will not be something great.
Objection 3: Further, every act of understanding means understanding something. When therefore God understands Himself, if He Himself is not distinct from this act of understanding, He understands that He understands Himself; and so on to infinity. Therefore the act of God's intellect is not His substance.
On the contrary, Augustine says (De Trin. vii), "In God to be is the same as to be wise." But to be wise is the same thing as to understand. Therefore in God to be is the same thing as to understand. But God's existence is His substance, as shown above ( Question [3], Article [4]). Therefore the act of God's intellect is His substance.
I answer that, It must be said that the act of God's intellect is His substance. For if His act of understanding were other than His substance, then something else, as the Philosopher says (Metaph. xii), would be the act and perfection of the divine substance, to which the divine substance would be related, as potentiality is to act, which is altogether impossible; because the act of understanding is the perfection and act of the one understanding. Let us now consider how this is. As was laid down above ( Article [2]), to understand is not an act passing to anything extrinsic; for it remains in the operator as his own act and perfection; as existence is the perfection of the one existing: just as existence follows on the form, so in like manner to understand follows on the intelligible species. Now in God there is no form which is something other than His existence, as shown above ( Question [3]). Hence as His essence itself is also His intelligible species, it necessarily follows that His act of understanding
