3 How then did the Lord answer those that were marvelling how He knew letters which He had not learned? “My doctrine,” says He, “is not mine, but His that sent me.” This is the first profundity. For He seems as if in a few words He had spoken contraries. For He says not, This doctrine is not mine; but, “My doctrine is not mine.” If not Yours, how Yours? If Yours, how not Yours? For You say both: both, “my doctrines;” and, “not mine.” For if He had said, This doctrine is not mine, there would have been no question.
But now, brethren, in the first place, consider well the question, and so in due order expect the solution. For he who sees not the question proposed, how can he understand what is expounded? The subject of inquiry, then, is that which He says, “My, not mine” this appears to be contrary; how “my,” how “not mine”? If we carefully look at what the holy evangelist himself says in the beginning of his Gospel, “In the beginning was the Word, and the Word was with God, and the Word was God;” thence hangs the solution of this question.
What then is the doctrine of the Father, but the Father's Word? Therefore, Christ Himself is the doctrine of the Father, if He is the Word of the Father. But since the Word cannot be of none, but of some one, He said both “His doctrine,” namely, Himself, and also, “not His own,” because He is the Word of the Father. For what is so much “Yours” as Yourself? And what so much not Yours as Yourself, if that You are is of another?
Source: Tractates on the Gospel of John (New Advent)