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 (Question 1, Article 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.
Source: Summa Theologica (New Advent)