When refuting the Gnostics, who pride themselves on knowledge of the divine, and make a pantheon out of such knowledge, Irenaeus expounds on the relation between a rational being and his knowledge, by way of proving that Christ, the Wisdom of God, is one with the Father, though distinct from the Father. This is the Doctrine of the Trinity, although Irenaeus doesn’t use the term explicitly.
The Unfolding of the Thought of Man
For the first exercise of [thought] respecting anything, is styled Ennœa; but when it continues, and gathers strength, and takes possession of the whole soul, it is called Enthymesis. This Enthymesis, again, when it exercises itself a long time on the same point, and has, as it were, been proved, is named Sensation. And this Sensation, when it is much developed, becomes Counsel. The increase, again, and greatly developed exercise of this Counsel becomes the Examination of thought (Judgment); and this remaining in the mind is most properly termed Logos (reason), from which the spoken Logos proceeds. But all the which have been mentioned are one and the same, receiving their origin from Nous, and obtaining appellation according to their increase.
Just as the human body, which is at one time young, then in the prime of life, and then old, has received appellations according to its increase and continuance, but not according to any change of substance, or on account of any loss of body, so is it with those. For, when one contemplates anything, he also thinks of it; and when he thinks of it, he has also knowledge regarding it; and when he knows it, he also considers it; and when he considers it, he also mentally handles it; and when he mentally handles it, he also speaks of it. But, as I have already said, it is Nous who governs all these, while He is himself invisible, and utters speech of himself by means of those processes which have been mentioned, as it were by rays, but He himself is not sent forth by any other.
These things may properly be said to hold good in men, since they are compound by nature, and consist of a body and a soul.
The Unity of the Thought of God
God is not as men are; and His thoughts are not like the thoughts of men. For the Father of all is at a vast distance from those affections and passions which operate among men. He is a simple, uncompounded Being, without diverse members, and altogether like, and equal to himself, since He is wholly understanding, and wholly spirit, and wholly thought, and wholly intelligence, and wholly reason, and wholly hearing, and wholly seeing, and wholly light, and the whole source of all that is good.
For He may well and properly be called an Understanding which comprehends all things, but He is not like the understanding of men; and He may most properly be termed Light, but He is nothing like that light with which we are acquainted. And so, in all other particulars, the Father of all is in no degree similar to human weakness. He is spoken of in these terms according to the love [we bear Him]; but in point of greatness, our thoughts regarding Him transcend these expressions. If then, even in the case of human beings, understanding itself does not arise from emission, nor is that intelligence which produces other things separated from the living man, while its motions and affections come into manifestation, much more will the mind of God never by any means be separated from Himself; nor can anything be produced as if by a different Being.
The Word of God
For no one can know the Father, unless through the Word of God, that is, unless by the Son revealing [Him]; neither can he have knowledge of the Son, unless through the good pleasure of the Father. But the Son performs the good pleasure of the Father; for the Father sends, and the Son is sent, and comes. And His Word knows that His Father is, as far as regards us, invisible and infinite; and since He cannot be declared [by any one else], He does Himself declare Him to us; and, on the other hand, it is the Father alone who knows His own Word. And both these truths has our Lord declared. Wherefore the Son reveals the knowledge of the Father through His own manifestation. For the manifestation of the Son is the knowledge of the Father; for all things are manifested through the Word. In order, therefore, that we might know that the Son who came is He who imparts to those believing on Him a knowledge of the Father, He said to His disciples: “No man knoweth the Son but the Father, nor the Father but the Son, and those to whomsoever the Son shall reveal Him;” thus setting Himself forth and the Father as He is, that we may not receive any other Father, except Him who is revealed by the Son.