45 Unless we regard these words as an integral part of His statement, we do them violence by forcing upon them an arbitrary and unbelieving interpretation. But if His answer refers to the grounds of their anger, our faith expresses rightly what He meant to teach, and the perversity of the ungodly is left without support for its profane delusion. Let us see then whether this reply is suitable to an accusation of working on the Sabbath. The Son can do nothing of Himself, but what He has seen the Father doing.
He has said just above, My Father works even until now, and I work. If by virtue of the authority of the Father's nature within Him, all that He works, He works with the Father in Him, and the Father works even until now on the Sabbath, then the Son, Who pleads the authority of the Father's working, is acquitted of blame. For the words, can do nothing, refer not to strength but to authority; He can do nothing of Himself, except what He has seen. Now, to have seen does not confer the power to do, and therefore He is not weak, if He can do nothing without having seen, but His authority is shown to depend on seeing.
Again the words, unless He has seen, signify the consciousness derived from seeing, as when He says to the Apostles, Behold I say unto you, Lift up your eyes, and look on the fields, that they are white already unto harvest. With the consciousness that the Father's nature is abiding in Him, and working in Him when He works, to forestall the idea that the Lord of the Sabbath has violated the Sabbath, He pronounces that, The Son can do nothing of Himself, but what He has seen the Father doing. And thus He demonstrates that His every action springs from His consciousness of the nature working within Him; when He works on the Sabbath, the Father works even until now on the Sabbath.
In what follows, however, He refers to the second cause of their indignation, For what things soever He does, the Son does in like manner. Is it false that, what things soever the Father does, the Son does in like manner? Does the Son of God admit a distinction between the Father's power and working and His own? Does He shrink from claiming the equality of homage befitting an equal in power and nature? If He does, disdain His weakness, and degrade Him from equality of nature with the Father.
But He Himself says only a little later, That all may honour the Son, even as they honour the Father, He that honours not the Son, honours not the Father which sent Him. Discover, if you can, the inferiority, when Both are equal in honour; make out the weakness, when Both work with the same power.
Source: On the Trinity (New Advent)