On the Holy Trinity

⁵ That which is accidental commonly implies that it can be lost by some change of the thing to which it is an accident. For although some accidents are said to be inseparable, which in Greek are called ἀχώριστα, as the color black is to the feather of a raven; yet the feather loses that color, not indeed so long as it is a feather, but because the feather is not always. Wherefore the matter itself is changeable; and whenever that animal or that feather ceases to be, and the whole of that body is changed and turned into earth, it loses certainly that color also.

Although the kind of accident which is called separable may likewise be lost, not by separation, but by change; as, for instance, blackness is called a separable accident to the hair of men, because hair continuing to be hair can grow white; yet, if carefully considered, it is sufficiently apparent, that it is not as if anything departed by separation away from the head when it grows white, as though blackness departed thence and went somewhere and whiteness came in its place, but that the quality of color there is turned and changed.

Therefore there is nothing accidental in God, because there is nothing changeable or that may be lost. But if you choose to call that also accidental, which, although it may not be lost, yet can be decreased or increased—as, for instance, the life of the soul: for as long as it is a soul, so long it lives, and because the soul is always, it always lives; but because it lives more when it is wise, and less when it is foolish, here, too, some change comes to pass, not such that life is absent, as wisdom is absent to the foolish, but such that it is less—nothing of this kind, either, happens to God, because He remains altogether unchangeable.