Summa contra Gentiles1.46 Chapter XLVI. That God understands by nothing else than his essence

From what has been proved above it is made evident that the divine intellect understands by no other intelligible species but the divine essence.

For the intelligible species is the formal principle of the intellectual operation; even as the form of every agent is the principle of that agent's proper operation. Now the intellectual operation of God is His essence, as we have shown. Wherefore something else would be the principle and cause of the divine essence, if the divine intellect understood by some intelligible species other than His essence: and this is in contradiction with what has been shown above.

Again. The intellect is made actually intelligent by the intelligible species: just as sense is made actually sentient by the sensible species. Hence the intelligible species is compared to the intellect as act to potentiality. And consequently if the divine intellect were to understand by a species other than itself, it would be in potentiality with respect to something: and this is impossible, as we have proved above.

Moreover. An intelligible species that is accessory to the essence of the intellect in which it is, has an accidental being: for which reason our knowledge is reckoned among the accidents. Now in God there can be no accident, as proved above. Therefore there is no species in His intellect besides the divine essence.

Further. An intelligible species is the image of something understood. Wherefore if in the divine intellect there be an intelligible species besides its essence, it will be the image of something understood. Either, therefore, it will be the image of the divine essence or of some other thing. But it cannot be the image of the divine essence: for then the divine essence would not be intelligible by itself, and this species would make it intelligible. Nor again can there be in the divine intellect a species distinct from its essence and representative of some other thing. For this image would be imprinted thereon by something. Not however by the divine intellect itself, because then the same thing would be agent and patient: and there would be an agent which imprints not its own but another's image on the patient, and thus not every agent would produce its like. Nor again by another: for then there would be an agent previous to the divine intellect. Therefore there cannot possibly be in it an intelligible species besides its essence.

Moreover. God's act of intelligence is His essence, as we have proved. Therefore if He understood by a species that is not His essence, it would be by something other than His essence. But this is impossible. Therefore He does not understand by a species that is not His essence.