In the former discourse, so far as the subject impressed us, and so far as our poverty of understanding attained to, we have spoken by occasion of the words of the Gospel, where it is written: “The Son cannot do anything of Himself, but what He sees the Father doing,”— what it is for the Son— that is, the Word, for the Son is the Word— “to see;” and as all things were made by the Word, how it is to be understood that the Son first sees the Father doing, and then only Himself also does the things which He has seen done, seeing that the Father has done nothing except by the Son. For all things were made by Him, and without Him was nothing made. We have not, however, delivered to you anything as fully explained, and that because we have not understood anything thus clearly set forth. For, indeed, speech sometimes fails even where the understanding makes way; how much more does speech suffer defect, where the understanding has nothing perfect! Now, therefore, as the Lord gives us, let us briefly run over the passage, and even today complete the due task. Should there perchance remain somewhat of time or of strength, we will reconsider (so far as it may be practicable for us and with you) what it is for the Word “to see” and “to be shown to;” since, in fact, all that is here spoken is such that, if understood according to man's sense, carnally, the soul full of vain fancies makes for us only certain images of the Father and the Son, just as of two men, the one showing, the other seeing; the one speaking, the other hearing—all which are idols of the heart. And if now at length idols have been cast down from their own temples, how much more ought they to be cast down from Christian hearts!
2. “The Son,” says He, “cannot do anything of Himself, but what He sees the Father doing.” This is true: hold this fast, while at the same time you do not let slip what you have gotten in the beginning of the Gospel, that “in the beginning was the Word, and the Word was with God, and the Word was God,” and especially that “all things were made by Him.” Join this that you have now heard to that hearing, and let both agree together in your hearts. Thus, “The Son cannot of Himself do anything, except what He sees the Father doing,” is yet in such wise that what the Father does, He does only by the Son, because the Son is His Word: and, “In the beginning was the Word, and the Word was with God, and the Word was God;” also, “All things were made by Him.” For what things soever He does, the Son also does in like manner; not other things, but these and not in a different, but in like manner.
3. “For the Father loves the Son, and shows Him all things that Himself does.” To that which He said above, “except what He sees the Father doing,” seems to belong this also, “He shows Him all things that Himself does.” But if the Father does show what He does, and the Son cannot do except the Father has shown, and if the Father cannot show unless He has done, it will follow that it is not through the Son that the Father does all things; moreover, if we hold it fixed and unshaken, that the Father does all by the Son, then He shows the Son before He does. For if the Father does show to the Son after He has done, that the Son may do the things shown, which being shown were already done, then doubtless something there is that the Father does without the Son. But the Father does not anything without the Son, because the Son of God is God's Word, and all things were made by Him. It remains, then, that possibly what the Father is about to do, He shows as about to be done, that it may be done by the Son. For if the Son does those things which the Father shows as already done, surely it is not by the Son that the Father has done the things which He thus shows. For they could not be shown to the Son unless they were first done, and the Son would not be able to do them unless they were first shown; therefore were they made without the Son. But yet it is a true thing, “All things were made by Him;” therefore they were shown before they were made. But this we said must be put off, and returned to after briefly scanning the passage, if, as we said, some portion of time and of strength should remain to us for a reconsideration of the matters deferred.
Source: Tractates on the Gospel of John (New Advent)