Summa contra Gentiles1.98 Chapter XCVIII. That God is his own life

From this it further appears that God is His own life.

For life in a living being is the same as to live expressed in the abstract; just as a running is in reality the same as to run. Now in living things to live is to be, as the Philosopher declares (2 De Anima). For since an animal is said to be living because it has a soul whereby it has existence, as it were by its proper form, it follows that to live is nothing but a particular kind of existence resulting from a particular kind of form. Now God is His own existence, as proved above. Therefore He is His own living and His own life.

Again. Intelligence is a kind of life, as the Philosopher declares (2 De Anima): since to live is the act of a living being. Now God is His own act of intelligence, as we have proved. Therefore He is His own living and His own life.

Moreover. If God were not His own life, since He is a living being as shown above, it would follow that He is living by a participation of life. Now whatever is by participation is reduced to that which is by its essence. Wherefore God would be reduced to something preceding Him whereby He would live. But this is impossible, as is evident from what has been said.

Again. If God is a living being, as we have proved, it follows that life is in Him. Wherefore if He be not His own life, there will be something in Him that is not Himself: and consequently He will be composite. But this was disproved above. Therefore God is His own life. This is What is said (Jo. xiv. 6): am the life.