5 He appears to have made Himself as it were less, when He said, “The Son cannot of Himself do anything, but what He sees the Father doing.” Hereupon heretical vanity lifts the neck; theirs, indeed, who say that the Son is less than the Father, of less authority, of less majesty, of less possibility, not understanding the mystery of Christ's words. But attend, beloved, and see how they are confounded in their carnal intellect by the words of Christ. And this is what I said a little before, that the word of God troubles all perverse hearts, just as it exercises pious hearts, especially that spoken by the Evangelist John.
For they are deep words that are spoken by him, not random words, nor such as may be easily understood. So, a heretic, if he happen to hear these words, immediately rises and says to us, “Lo, the Son is less than the Father; hear the words of the Son, who says, 'The Son cannot do anything of Himself, but what He sees the Father doing.'” Wait; as it is written, “Be meek to hear the word, that you may understand.” Well, suppose that because I assert the power and majesty of the Father and of the Son to be equal, I was disconcerted at hearing these words, “The Son cannot do anything of Himself, but what He sees the Father doing.”
Well, I, being disconcerted at these words, will ask you, who seemest to yourself to have instantly understood them, a question. We know in the Gospel that the Son walked upon the sea; when saw He the Father walk upon the sea? Here now he is disconcerted. Lay aside, then, your understanding of the words, and let us examine them together. What do we then? We have heard the words of the Lord: “The Son cannot of Himself do anything, but what He sees the Father doing.” The Son walked upon the sea, the Father never walked upon the sea. Yet certainly “the Son cannot of Himself do anything, but what He sees the Father doing.”
Source: Tractates on the Gospel of John (New Advent)