Chapter 2.--God the Only Unchangeable Essence.
3. He is, however, without doubt, a substance, or, if it be better so to call it, an essence, which the Greeks call ousia. For as wisdom is so called from the being wise, and knowledge from knowing; so from being [566] comes that which we call essence. And who is there that is, more than He who said to His servant Moses, "I am that I am;" and, "Thus shall thou say unto the children of Israel, He who is hath sent me unto you?" [567] But other things that are called essences or substances admit of accidents, whereby a change, whether great or small, is produced in them. But there can be no accident of this kind in respect to God; and therefore He who is God is the only unchangeable substance or essence, to whom certainly being itself, whence comes the name of essence, most especially and most truly belongs. For that which is changed does not retain its own being; and that which can be changed, although it be not actually changed, is able not to be that which it had been; and hence that which not only is not changed, but also cannot at all be changed, alone falls most truly, without difficulty or hesitation, under the category of being.
[566] Esse
[567] Ex. iii. 14
Chapter 3.--The Argument of the Arians is Refuted, Which is Drawn from the Words Begotten and Unbegotten.
4. Wherefore,--to being now to answer the adversaries of our faith, respecting those things also, which are neither said as they are thought, nor thought as they really are:--among the many things which the Arians are wont to dispute against the Catholic faith, they seem chiefly to set forth this, as their most crafty device, namely, that whatsoever is said or understood of God, is said not according to accident, but according to substance, and therefore, to be unbegotten belongs to the Father according to substance, and to be begotten belongs to the Son according to substance; but to be unbegotten and to be begotten are different; therefore the substance of the Father and that of the Son are different. To whom we reply, If whatever is spoken of God is spoken according to substance, then that which is said, "I and the Father are one," [568] is spoken according to substance. Therefore there is one substance of the Father and the Son. Or if this is not said according to substance, then something is said of God not according to substance, and therefore we are no longer compelled to understand unbegotten and begotten according to substance. It is also said of the Son, "He thought it not robbery to be equal with God." [569] We ask, equal according to what? For if He is not said to be equal according to substance, then they admit that something may be said of God not according to substance. Let them admit, then, that unbegotten and begotten are not spoken according to substance. And if they do not admit this, on the ground that they will have all things to be spoken of God according to substance, then the Son is equal to the Father according to substance.
[568] John x. 30
[569] Phil. ii. 6
Chapter 4.--The Accidental Always Implies Some Change in the Thing.
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 achorista, 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.