On The Holy Trinity by St. Augustine
Chapter 6.--Reply is Made to the Cavils of the Heretics in Respect to the Same Words Begotten and Unbegotten.
7. But if they think they can answer this reasoning thus, -- that the Father indeed is so called in relation to the Son, and the Son in relation to the Father, but that they are said to be unbegotten and begotten in relation to themselves, not in relation each to the other; for that it is not the same thing to call Him unbegotten as it is to call Him the Father, because there would be nothing to hinder our calling Him unbegotten even if He had not begotten the Son; and if any one beget a son, he is not therefore himself unbegotten, for men, who are begotten by other men, themselves also beget others; and therefore they say the Father is called Father in relation to the Son, and the Son is called Son in relation to the Father, but unbegotten is said in relation to Himself, and begotten in relation to Himself; and therefore, if whatever is said in relation to oneself is said according to substance, while to be unbegotten and to be begotten are different, then the substance is different: -- if this is what they say, then they do not understand that they do indeed say something that requires more careful discussion in respect to the term unbegotten, because neither is any one therefore a father because unbegotten, nor therefore unbegotten because he is a father, and on that account he is supposed to be called unbegotten, not in relation to anything else, but in respect to himself; but, on the other hand, with a wonderful blindness, they do not perceive that no one can be said to be begotten except in relation to something. For he is therefore a son because begotten; and because a son, therefore certainly begotten. And as is the relation of son to father, so is the relation of the begotten to the begetter; and as is the relation of father to son, so is the relation of the begetter to the begotten. And therefore any one is understood to be a begetter under one notion, but understood to be unbegotten under another. For though both are said of God the Father, yet the former is said in relation to the begotten, that is to the Son, which, indeed, they do not deny; but that He is called unbegotten, they declare to be said in respect to Himself. They say then, If anything is said to be a father in respect to itself, which cannot be said to be a son in respect to itself, and whatever is said in respect to self is said according to substance; and He is said to be unbegotten in respect to Himself, which the Son cannot be said to be; therefore He is said to be unbegotten according to substance; and because the Son cannot be so said to be, therefore He is not of the same substance. This subtlety is to be answered by compelling them to say themselves according to what it is that the Son is equal to the Father; whether according to that which is said in relation to Himself, or according to that which is said in relation to the Father. For it is not according to that which is said in relation to the Father, since in relation to the Father He is said to be Son, and the Father is not Son, but Father. Since Father and Son are not so called in relation to each other in the same way as friends and neighbors are; for a friend is so called relatively to his friend, and if they love each other equally, then the same friendship is in both; and a neighbor is so called relatively to a neighbor, and because they are equally neighbors to each other (for each is neighbor to the other, in the same degree as the other is neighbor to him), there is the same neighborhood in both. But because the Son is not so called relatively to the Son, but to the Father, it is not according to that which is said in relation to the Father that the Son is equal to the Father; and it remains that He is equal according to that which is said in relation to Himself. But whatever is said in relation to self is said according to substance: it remains therefore that He is equal according to substance; therefore the substance of both is the same. But when the Father is said to be unbegotten, it is not said what He is, but what He is not; and when a relative term is denied, it is not denied according to substance, since the relative itself is not affirmed according to substance.