(CG 1/046). That God understands by nothing else than by His own Essence (Book 1: Of God As He Is In Himself) (Summa Contra Gentiles) (Aquinas, Thomas, St.)

WHEN by an impression on the understanding that power is brought to bear on its object, the perfection of the intellectual act depends on two things: one is the perfect conformity of the impression with the thing understood: the other is the perfect fixing of the impression on the understanding: which perfection is the greater, the greater the power of the understanding to understand. Now the mere divine essence, which is the intelligible representation whereby the divine understanding understands, is absolutely one and the same with God Himself and with the understanding of God. God therefore knows Himself most perfectly.

6. The perfections of all creatures are found at their best in God. But of perfections found in creatures the greatest is to understand God: seeing that the intellectual nature is pre-eminent above other natures, and the perfection of intellect is the act of understanding, and the noblest object of understanding is God. God therefore understands Himself perfectly.

This also is confirmed by divine authority, for the Apostle says: The spirit of God searcheth into even the deep things of God (i Cor. ii, 10).