8 Wherefore the works of the Father and of the Son are inseparable. Moreover, this, “The Son cannot do anything of Himself,” would mean the same thing as if He were to say, “The Son is not from Himself.” For if He is a Son, He was begotten; if begotten, He is from Him of whom He is begotten. Nevertheless, the Father begot Him equal to Himself. Nor was anything wanting to Him that begot; He who begot a co-eternal required not time to beget: who produced the Word of Himself, required not a mother to beget by; the Father begetting did not precede the Son in age, so that He should beget a Son younger than Himself.
But perhaps some one may say, that after many ages God begot a Son in His old age. Even as the Father is without age, so the Son is without growth; neither has the one grown old nor the other increased, but equal begot equal, eternal begot eternal. How, says some one, has eternal begot eternal? As a temporary flame generates a temporary light. The generating flame is coeval with the light which it generates: the generating flame does not precede in time the generated light; but from the moment the flame begins, from that moment the light begins.
Show me flame without light, and I show you God the Father without Son. Accordingly, “the Son cannot do anything of Himself, but what He sees the Father doing,” implies, that for the Son to see and to be begotten of the Father, is the same thing. His seeing and His substance are not different; nor are His power and substance different. All that He is, He is of the Father; all that He can is of the Father; because what He can and what He is is one thing, and all of the Father.
Source: Tractates on the Gospel of John (New Advent)