Summa contra Gentiles1.18 Chapter XVIII. That in God there is no composition

From the foregoing we are able to conclude that there is no composition in God. For in every composite thing there must needs be act and potentiality: since several things cannot become one simply, unless there be something actual there and something else potential. Because those things that are actually, are not united except as an assemblage or group, which are not one simply. In these moreover the very parts that are gathered together are as a potentiality in relation to the union: for they are actually united after being potentially unitable. But in God there is no potentiality. Therefore in Him there is no composition.

Again. Every composite is subsequent to its components. Therefore the first being, namely God, has no component parts.

Further. Every composite is potentially dissoluble, so far as its composite nature is concerned, although in some there is something else incompatible with dissolution. Now that which is dissoluble is in potentiality to not-being.

But this cannot be said of God, since of His very essence He is necessarily. Therefore there is no composition in Him. Moreover. Every composition requires a compounder: for if there be composition, it results from several things: and things that are several in themselves would not combine together unless they were united by a compounder. If then God were composite, He would have a compounder: for He could not compound Himself, since no thing is its own cause, for it would precede itself, which is impossible. Now the compounder is the efficient cause of the composite. Therefore God would have an efficient cause: and thus He would not be the first cause, which was proved above.

Again. In any genus the more simple a thing is the more excellent it is; such, in the genus hot, is fire which has no admixture of cold. Therefore that which obtains the summit of nobility among beings, must be in the summit of simplicity. Now that which obtains the summit of nobility in things is what we call God, since He is the first cause, because the cause is more excellent than its effect. Therefore there can be no composition in Him.

Moreover. In every composite thing the good does not belong to this or that part but to the whole, and I speak of good in reference to that goodness which is proper to, and is the perfection of, the whole: thus the parts are imperfect in relation to the whole: thus the parts of a man are not a man, nor have the parts of the number six the perfection of six, nor do the parts of a line attain to the perfection of the measure found in the whole line. Therefore if God is composite, His proper perfection and goodness are found in the whole of God but not in any of His parts. And thus the good that is proper to Him will not be purely in Him; and consequently He will not be the first and supreme good.

Further. Before every multitude it is necessary to find unity. Now in every composite there is multitude. Therefore that which is before all things, namely God, must needs be devoid of all composition.