[The following is an extract from Thomas Aquinas’ Summa Theologiae: Volume 2, Existence and Nature of God: 1a. 2-11 (Cambridge University Press, 2006), pp.89-90]
“For to be called ‘good’ a thing must be perfect. Now there is a threefold perfection in things: firstly, they are established in existence; secondly, they possess in addition certain accidents necessary to perfect their activity; and a third perfection comes when they attain some extrinsic goal. Thus the primary perfection of fire lies in existing according to its own substantial form, a secondary perfection consists in heat, lightness, dryness, and so on; and a third perfection is being at rest in its appropriate place.
Now this threefold perfection belongs by nature to no caused thing, but only to God; for he alone exists by nature, and in him there are no added accidents (power, wisdom and the like which are accidental to other things belonging to him by nature, as already noted). Moreover, he is not disposed towards some extrinsic goal, but is himself the ultimate goal of all other things. So it is clear that only God possess every kind of perfection by nature. He alone therefore is by nature good.
Hence: 1. Being one does not involve being perfect, but only being undivided, and things belongs to everything by nature. For the natures of simple things are both undivided and indivisible, and the natures of composite things are at least undivided. So things whilst necessarily one by nature, are not, as we have shown, necessarily good by nature.
2. Although things are good inasmuch as they exist, nevertheless existence is not the nature of any created thing, and so it does not follow that created things are good by nature.
3. The goodness of a created thing is not its nature, but something additional: either its existence, or some added perfection, or some relatedness to a goal. This additional goodness however is said to be good in the same way that it is said to exist. Now it is said to exist as a mode which something exists, not as something having its own mode of existence. And so it is said to be good because things that possess it are good, not because it itself possess some other goodness making it good.”