Commentaria in Summam contra Gentiles, Liber I

Second, he determines concerning the divine life with respect to what it is¹. And this conclusion is set down: God is His own life. And thus the divine life is the same as His essence.

1. It is proved first. God is his own esse. Therefore he is his life. — The consequence is proved. Because life is the very act of living signified in abstraction, just as running is the very act of running. But the living of living things is their esse, as is said in II de Anima, text 37.

Note that life and to live, as is stated in the First Part, q. 54, a. 1, ad 2, are not related as essence and esse: rather, life sometimes signifies the essence of things that move themselves; sometimes, however, it signifies the very esse of a thing having such an essence. And in both ways life and to live are related as running and to run. For running and to run signify the same thing, namely the act of running, but running signifies the act in the manner of an abstract term, while to run signifies it in the manner of a concrete one. Similarly, life, insofar as it signifies essence, and to live signify the same thing, but life in the abstract, and to live in the concrete: for life signifies absolutely the essence of a self-mover, while to live signifies having such an essence, as Saint Thomas seems to intend in the First Part, q. 18, a. 2. Life also, in its second signification, signifies the same thing as living itself, for it signifies the act of being of that to which it belongs to move itself, namely in relation to the vital principle: but life signifies this in the manner of an abstract term, for it signifies the very esse of the living thing absolutely; while to live does so in the manner of a concrete term, for it signifies having the esse of a self-mover. — Saint Thomas therefore takes the name "life" in its second signification, not in the first.

But it must be noted, according to the doctrine of St. Thomas (Quodlibet IX, a. 3, ad 1), that to live, insofar as it is taken for esse, does not signify esse absolutely and precisely, but connotes the principle of essendi; for it signifies esse as specified in some way by some principle of essendi. Therefore, although only one esse of the supposit is posited in Christ, nevertheless a twofold living is posited: for the very esse of the divine supposit, as it subsists in the divine nature, is divine living; but insofar as that esse is that of the supposit subsisting in human nature, it is human living.

II. Second. God is his own understanding. Therefore also his own living. — The consequence is evident, because understanding is a kind of living, as is said in II de Anima, text 13.

It must be noted that "to understand" can be said to be "to live" in two ways: in one way, as living names the operation of a living thing, which is less properly called life; in another way, as it names the essence or esse of a living thing. If it is taken in the first way, this argument proves that the life which is the operation of a living thing is in God the same as God. If it is taken in the second way, it is thus proved that life taken properly, which is the very esse or the very essence, is the same as God. For if God is his own understanding, and his understanding is his esse or his essence, which is his life and his living, it follows that he is his own life. In both ways the conclusion is true, and it can be understood in both ways. From the first sense we have that not only is God his own life, which is his essence and his esse, as the first argument concluded: but he is also his own vital operation. The second way, however, seems to be more what was intended: because life is taken less properly for the operations of life, as is said in Part I ².

Third. It would follow that God would be living by participation in life. And thus he would be reduced to something prior to himself, which would be living of itself.

Fourth. It would follow that there would be something in him which is not himself. And thus he would be composed.

It is confirmed by John 14.