On The Holy Trinity by St. Augustine
Chapter 5.--Nothing is Spoken of God According to Accident, But According to Substance or According to Relation.
6. Wherefore nothing in Him is said in respect to accident, since nothing is accidental to Him, and yet all that is said is not said according to substance. For in created and changeable things, that which is not said according to substance, must, by necessary alternative, be said according to accident. For all things are accidents to them, which can be either lost or diminished, whether magnitudes or qualities; and so also is that which is said in relation to something, as friendships, relationships, services, likenesses, equalities, and anything else of the kind; so also positions and conditions, places and times, acts and passions. But in God nothing is said to be according to accident, because in Him nothing is changeable; and yet everything that is said, is not said, according to substance. For it is said in relation to something, as the Father in relation to the Son and the Son in relation to the Father, which is not accident; because both the one is always Father, and the other is always Son: yet not |always,| meaning from the time when the Son was born [natus], so that the Father ceases not to be the Father because the Son never ceases to be the Son, but because the Son was always born, and never began to be the Son. But if He had begun to be at any time, or were at any time to cease to be, the Son, then He would be called Son according to accident. But if the Father, in that He is called the Father, were so called in relation to Himself, not to the Son; and the Son, in that He is called the Son, were so called in relation to Himself, not to the Father; then both the one would be called Father, and the other Son, according to substance. But because the Father is not called the Father except in that He has a Son, and the Son is not called Son except in that He has a Father, these things are not said according to substance; because each of them is not so called in relation to Himself, but the terms are used reciprocally and in relation each to the other; nor yet according to accident, because both the being called the Father, and the being called the Son, is eternal and unchangeable to them. Wherefore, although to be the Father and to be the Son is different, yet their substance is not different; because they are so called, not according to substance, but according to relation, which relation, however, is not accident, because it is not changeable.