Summa contra Gentiles1.28 Chapter XXVIII. Of the divine perfection

Now although things that exist and live are more perfect than those which only exist, yet God Who is not distinct from His own existence, is universally perfect being. And by universally perfect I mean that He lacks not the excellence of any genus.

For every excellence of any being whatsoever is ascribed to a thing in respect of its being, since no excellence would accrue to man from his wisdom, unless thereby he were wise, and so on. Wherefore, according as a thing has being, so is its mode of excellence: since a thing, according as its being is contracted to some special mode of excellence more or less great, is said to be more or less excellent. Hence if there be a thing to which the whole possibility of being belongs, no excellence that belongs to any thing can be lacking thereto. Now to a thing which is its own being, being belongs according to the whole possibility of being: thus if there were a separate whiteness, nothing of the whole possibility of whiteness could be wanting to it: because something of the possibility of whiteness is lacking to a particular white thing through a defect in the recipient of whiteness, which receives it according to its mode and, maybe, not according to the whole possibility of whiteness. Therefore God, Who is His own being, as shown above, has being according to the whole possibility of being itself: and consequently He cannot lack any excellence that belongs to any thing.

And just as every excellence and perfection is in a thing according as that thing is, so every defect is in a thing according as that thing in some sense is not. Now just as God has being wholly, so is not-being wholly absent from Him, since according as a thing has being it fails in not-being. Therefore all defect is removed from God, and consequently He is universally perfect.

But those things which only exist are imperfect, not on account of an imperfection in absolute being itself, for they have not being according to its whole possibility, but because they participate being in a particular and most imperfect way.

Again. Every imperfect thing must needs be preceded by some perfect thing: for seed is from some animal or plant. Wherefore the first being must be supremely perfect. Now it has been shown that God is the first being. Therefore He is supremely perfect.

Moreover. A thing is perfect in so far as it is in act, and imperfect in so far as it is in potentiality and void of act. Wherefore that which is nowise in potentiality but is pure act, must needs be most perfect. Now such is God. Therefore He is most perfect.

Further. Nothing acts except according as it is in act: wherefore action follows upon the mode of actuality in the agent; and consequently it is impossible for the effect that results from an action to have a more excellent actuality than that of the agent, although it is possible for the actuality of the effect to be more imperfect than that of the active cause, since action may be weakened on the part of that in which it terminates. Now in the genus of efficient cause we come at length to the one cause which is called God, as explained above, from Whom all things proceed, as we shall show in the sequel. Wherefore it follows that whatever is actual in any other thing, is found in God much more eminently than in that thing, and not conversely. Therefore God is most perfect.

Again. In every genus there is some thing most perfect relatively to that genus, by which every thing in that genus is measured: since every thing is shown to be more or less perfect according as it approaches more or less to the measure of that genus: thus white is said to be the measure in all colours, and the virtuous among all men. Now the measure of all beings can be none other than God Who is His own being. Therefore no perfection that belongs to any thing is lacking to Him, otherwise He would not be the universal measure of all.

Hence it is that when Moses sought to see the face of God, the Lord answered him: will show thee all good (Exod. xxxiii. 18, 19), giving thus to understand that the fulness of all good is in Him. And Dionysius says (Div. Nom. v.): God exists not in any single mode, but embraces and prepossesses all being within Himself, absolutely and without limit.

It must however be observed that perfection cannot fittingly be ascribed to God if we consider the meaning of the word in respect of its derivation: since what is not made, cannot seemingly be described as perfect. Yet since whatever is made has been brought from potentiality to act, and from not-being to being, when it was made; it is rightly described as perfect, i.e., completely made, when its potentiality is completely reduced to act, so that it retains nothing of not-being, and has complete being. Accordingly by a kind of extension of the term, perfect is applied not only to that which has arrived at complete act through being made, but also to that which is in complete act without being made at all. It is thus that we say that God is perfect, according to Matt. v. 48: Be ye perfect as also your heavenly Father is perfect.