Summa contra Gentiles1.47 Chapter XLVII. That God understands himself perfectly

It is furthermore clear from the above that God understands Himself perfectly.

For since the intellect is directed by the intelligible species to the thing understood, the perfection of intellectual operation depends on two things. One is that the intelligible species be perfectly conformed to the thing understood. The other is that it be perfectly united to the intellect; and this is all the more so, according as the intellect is endowed with greater efficacy in understanding. Now the divine essence which is the intelligible species whereby the divine intellect understands, is absolutely the same as God Himself, and is altogether identified with His intellect. Therefore God understands Himself most perfectly.

Further. A material thing is made intelligible by being abstracted from matter and from material conditions. Wherefore that which by its nature is severed from matter and from material conditions, is by its very nature intelligible. Now every intelligible is understood according as it is actually one with the intelligent: and God is Himself intelligent, as we have proved. Therefore since He is altogether immaterial, and is absolutely one with Himself, He understands Himself most perfectly.

Again. A thing is actually understood through the unification of the intellect in act and the intelligible in act. Now the divine intellect is always intellect in act: since nothing is in potentiality and imperfect in God. And God's essence is by itself perfectly intelligible, as shown above. Since, then, the divine intellect and the divine essence are one, as stated above, it is evident that God understands Himself perfectly: for God is both His own intellect and His own essence.

Moreover. Whatever is in anyone in an intelligible manner, is understood by him. Now the divine essence is in God in an intelligible manner: for God's natural being and His intelligible being are one and the same, since His being is His act of intelligence. Therefore God understands His essence. Therefore He understands Himself, since He is His very essence.

Further. The acts of the intellect, as of the other powers of the soul, are distinguished according to their objects. Hence the more perfect the intelligible, the more perfect will the operation of the intellect be. Now the most perfect intelligible is the divine essence, since it is the most perfect act and the first truth. And the operation of the divine intellect is also the most excellent, since it is the divine being itself, as we have shown. Therefore God understands Himself.

Again. All the perfections of things are found eminently in God. Now among other perfections found in created things is that of understanding God: since the intellectual nature whose perfection it is to understand stands above others: and God is the most excellent intelligible. Therefore God, most of all, understands Himself.

This is confirmed by divine authority. For the Apostle says (1 Cor. ii. 10) that the Spirit of God searcheth... the deep things of God.