17 The chief reason why the Jews wished to kill the Lord was that, in calling God His Father, He had made Himself equal with God; and therefore He put His answer, in which He reproved their evil passion, into the form of an exposition of the whole mystery of our faith. For just before this, when He had healed the paralytic and they had passed their judgment upon Him that He was worthy of death for breaking the Sabbath, He had said, My Father works hitherto, and I work. Their jealousy had been inflamed to the utmost by the raising of Himself to the level of God which was involved in this use of the name of Father.
And now He wishes to assert His birth and to reveal the powers of His nature, and so He says, I say unto you, the Son can do nothing of Himself, but what He sees the Father do. These opening words of His reply are aimed at that wicked zeal of the Jews, which hurried them on even to the desire of slaying Him. It is in reference to the charge of breaking the Sabbath that He says, My Father works hitherto, and I work. He wished them to understand that His practice was justified by Divine authority; and He taught them by the same words that His work must be regarded as the work of the Father, Who was working in Him all that He wrought.
And again, it was to subdue the jealousy awakened by His speaking of God as His Father that He uttered those words, Verily, verily, I say unto you, the Son can do nothing of Himself, but what He sees the Father do. Lest this making of Himself equal to God, as having the name and nature of God's Son, should withdraw men's faith from the truth that He had been born, He says that the Son can do nothing but what He sees the Father do. Next, in confirmation of the saving harmony of truths in our confession of Father and of Son, He displays this nature which is His by birth; a nature which derives its power of action not from successive gifts of strength to do particular deeds, but from knowledge.
He shows that this knowledge is not imparted by the Father's performance of any bodily work, as a pattern, that the Son may imitate what the Father has previously done; but that, by the action of the Divine nature, He had come to share the subsistence of the Divine nature, or, in other words, had been born as Son from the Father. He told them that, because the power and the nature of God dwelt consciously within Him, it was impossible for Him to do anything which He had not seen the Father doing; that, since it is in the might of the Father that God the Only-begotten performs His works His liberty of action coincides in its range with His knowledge of the powers of the nature of God the Father; a nature inseparable from Himself, and lawfully owned by Him in virtue of His birth.
For God sees not after a bodily fashion, but possesses, by His nature, the vision of Omnipotence.
Source: On the Trinity (New Advent)