On The Holy Trinity by St. Augustine
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,| 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.| 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.