Summa contra Gentiles1.45 Chapter XLV. That God's act of intelligence is his essence

From the fact that God is intelligent it follows that His act of intelligence is His essence.

For intelligence is the act of an intelligent being, existing within that being and not passing on to something outside of it, as heating passes into the thing heated: for the intelligible suffers nothing through being understood, but the one who understands is perfected. Now whatever is in God is the divine essence. Therefore God's act of intelligence is the divine essence, the divine existence, and God Himself: since God is His essence and His existence.

Further. The act of intelligence is compared to the in- 1 Ps. cxxxviii. 6. 2 Ch. xxiii. 3 Chs. xxi., xxii.

tellect as existence to essence. But God's existence is His essence, as proved above. Therefore God's act of intelligence is His intellect. Now the divine intellect is God's essence, otherwise it would be accidental to God. Therefore the divine act of intelligence must needs be His essence.

Moreover. Second act is more perfect than first act, even as consideration is more perfect than knowledge. Now God's knowledge or intellect is His very essence, if He is intelligent as shown above: since no perfection belongs to Him by participation, but by essence, as already proved. If, therefore, His act of consideration be not His essence, something will be more noble and perfect than His essence. And thus He will not be in the summit of perfection and goodness: and consequently He will not be first.

Again. Intelligence is the act of the intelligent. If then God being intelligent is not His act of intelligence, He must be compared to it as potentiality to act: and so there will be potentiality and act in God; which is impossible, as we have proved above.

Again. Every substance is for the sake of its operation. If therefore God's operation is other than the divine substance, His end will be other than Himself. And thus God will not be His own goodness, since the good of a thing is its end.

If, however, God's act of intelligence is His existence, His act of intelligence must be simple, eternal, unchangeable, existing only in act, and all those things which have been proved about the divine existence. Wherefore God is not in potentiality to intelligence, nor does He begin to understand a thing anew, nor is His act of intelligence subject to any change or composition whatsoever.