Summa contra Gentiles1.73 Chapter LXXIII. That God's will is his essence

It is evident from the foregoing that His will is not distinct from His essence.

For it belongs to God to have a will in as much as He has an intellect, as proved above. Now He is intelligent by His essence, as we have already shown: and consequently will also is in Him by His essence. Therefore God's will is His very essence.

Again. Even as to understand is the perfection of one who is intelligent, so to will is the perfection of one who wills, for each is an action abiding in the agent, and not passing into something passive, as heating. Now God's act of intelligence is His being, as we proved above; because, since God's being is by itself supremely perfect, it admits of no additional perfection, as we have shown above. Therefore the divine willing is also His being: and consequently God's will is His essence.

Moreover. Since every agent acts in so far as it is actual, it follows that God, Who is pure act, acts by His essence. Now willing is an operation of God. Therefore it follows that God wills by His essence. Therefore His will is His essence.

Again. If will were something added to the divine substance, since the divine substance is complete in being, it would follow that will would be adventitious to Him like an accident to its subject; that the divine substance would be compared thereto as potentiality to act; and that there is composition in God. All of which have been disproved above. It is therefore impossible for the divine will to be something in addition to the divine essence.