We’re still on our quest to define what God is.
Chapter 28: Of The Divine Perfection
1 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.i
2 [This may be skipped.] 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.ii
3 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…iii
5 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.
6 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.iv
7 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.v
8 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…vi
iAs the song says, dead puppies aren’t much fun—and they are surely less excellent as puppies than live ones. Here is the crucial idea: the lack of the good is the lack of perfection. Which is to say (in brief), evil is the lack of the good, and the good is perfection. It’s not that God is lacking some thing, but since His essence and existence are the same, He is perfect: nothing could be added to make his essence more perfect. The word perfect does not here retain its popular meaning: it is a technical term.
iiI considered leaving this one out as it is, sort of, a more long-winded repeat of argument 1. But I left it because of this sentence: “…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.” This is an analogy. If whiteness could somehow per impossible exist on its own, it could not lack in whiteness, right? It would, in the sense of the word above, be perfectly white. Now things which are meant to be white, say a certain flower, which are somehow not white are that way due to some imperfection, an imperfection that cannot be caused by the perfection of whiteness. And the same with God being being-itself.
iiiIsn’t that pretty? Since God is being-itself, He cannot be in any part not-being, therefore he isn’t lacking that which the good of His essence demands. Thus He is perfect—in this sense. We still have lots of work to do to detail the consequences of this.
ivThese last two follow simply from what was proven before.
vCauses must be at least as great as their effects: causes are more excellent than effects. And we’re right back at Chapter 13, which is ever required reading. Don’t skimp here: review, review, review. Book 2, mentioned in footnote 6, is still about 75 chapters off.
viVirtue is what we’re aiming at; most of us, including Yours Truly, are lacking in this regard. But don’t miss the main point: “…since every thing is shown to be more or less perfect according as it approaches more or less to the measure of that genus”. Since imperfection abounds, it’s obvious that inequality is part of the (fallen) system. But since imperfection varies as the distance from the measure of a genus, we have natural goals, things to aim at. It is possible and necessary to measure ourselves against a standard. In other words, though one many may be more virtuous than another, goodness itself is not ultimately relative. We all aim at the same standard.
 Sum. Th. P. I., Q. iv., A. 2.
 Ch. xxii.
 Ch. xiii.
 Ch. xvi.
 Ch. xiii.
 Bk. II., ch. xv.
 3 Ethic. iv. 5; v. 10.