4 Nor need you wonder that the ineffable knowledge of God, whereby He is cognizant of all things, is, because of the various modes of human speech designated by the names of all those bodily senses; since even our own mind, in other words, the inner man—to which, while itself exercising its knowing faculty in one uniform way, the different subjects of its knowledge are communicated by those five messengers, as it were, of the body, when it understands, chooses, and loves the unchangeable truth—is said both to see the light, whereof it is said, “That was the true light;” and to hear the word, whereof it is said, “In the beginning was the Word;” and to be susceptible of smell, of which it is said, “We will run after the smell of your ointments;” and to drink of the fountain, whereof it is said, “With You is the fountain of life;” and to enjoy the sense of touch, when it is said, “But it is good for me to cleave unto God;” in all of which it is not different things, but the one intelligence, that is expressed by the names of so many senses.
When, therefore, it is said of the Holy Spirit, “For He shall not speak of Himself; but whatsoever He shall hear, that shall He speak,” so much the more is a simple nature, which is simple [uncompounded] in the truest sense, to be either understood or believed, which in its extent and sublimity far surpasses the nature of our minds. For there is mutability in our mind, which comes by learning to the perception of what it was previously ignorant of, and loses by unlearning what it formerly knew; and is deceived by what has a similarity to truth, so as to approve of the false in place of the true, and is hindered by its own obscurity as by a kind of darkness from arriving at the truth.
And so that substance is not in the truest sense simple, to which being is not identical with knowing; for it can exist without the possession of knowledge. But it cannot be so with that divine substance, for it is what it has. And on this account it has not knowledge in any such way as that the knowledge whereby it knows should be to it one thing, and the essence whereby it exists another; but both are one. Nor ought that to be called both, which is simply one. “As the Father has life in Himself,” and He Himself is not something different from the life that is in Him; “so has He given to the Son to have life in Himself,” that is, has begotten the Son, that He also should Himself be the life.
Accordingly we ought to accept what is said of the Holy Spirit, “For he shall not speak of Himself; but whatsoever He shall hear, that shall He speak,” in such a way as to understand thereby that He is not of Himself. Because it is the Father only who is not of another. For the Son is born of the Father, and the Holy Spirit proceeds from the Father; but the Father is neither born of, nor proceeds from, another. And yet surely there should not on that account occur to human thought any idea of disparity in the supreme Trinity; for both the Son is equal to Him of whom He is born, and the Holy Spirit to Him from whom He proceeds.
But what difference there is in such a case between proceeding and being born, would be too lengthy to make the subject of inquiry and dissertation, and would make our definition liable to the charge of rashness, even after we had discussed it; for such a thing is of the utmost difficulty, both for the mind to comprehend in any adequate way, and even were it so that the mind has attained to any such comprehension, for the tongue to explain, however able the one that presides as a teacher, or he that is present as a hearer.
Accordingly, “He shall not speak of Himself;” because He is not of Himself. “But whatsoever He shall hear, that shall He speak:” He shall hear of Him from whom He proceeds. To Him hearing is knowing; but knowing is being, as has been discussed above. Because, then, He is not of Himself, but of Him from whom He proceeds, and of whom He has essence, of Him He has knowledge; from Him, therefore, He has hearing, which is nothing else than knowledge.
Source: Tractates on the Gospel of John (New Advent)