8 Behold, again we humble ourselves to carnal notions, and descend to you, if indeed we had at any time ascended somewhat from you. You wish to show something to your son, that he may do what you do, you are about to do, and thus to show the thing. Therefore, what you are about to do, in order to show it to your son, you do not surely by your son; but you alone do that thing which, when done, he may see, and do another such thing in like manner. This is not the case there; why do you go on to your own similitude, and blottest out the similitude of God within you?
There, the case is wholly otherwise. Find a case in which you show to your son what you do before you do it; so that, after you have shown it, it will be by the son you do. Perhaps something like this now occurs to you: Lo, do you say, I think to make a house, and I wish it to be built by my son: before I build it myself, I point out to my son what I mean to do: both he does, and I too by him to whom I pointed out my wish. You have retreated, indeed, from the former similitude, but still you lie in great dissimilitude.
For, lo, before you can make the house, you inform your son, and point out to him what you mean to do; that, upon your showing before you make, he may make what you have shown, and so you may make by him: but you will speak words to your son, words will have to pass between you and him; between the person showing and the person seeing, between speaker and hearer, flies articulate sound, which is not what you are, nor what he is. That sound, indeed, which goes out of your mouth, and by the concussion of the air touches your son's ear, and filling the sense of hearing, conveys your thought to his heart; that sound, I say, is not yourself, nor your son.
A sign is given from your mind to your son's mind, but that sign not either your mind or your son's mind, but something else. Is it thus that we think the Father has spoken to the Son? Were there words between the Father and the Word? Then how is it? Or, whatever the Father would say to the Son, if He would say it by a word, the Son Himself is the Word of the Father, would He speak by a word to the Word? Or, since the Son is the great Word, had smaller words to pass between the Father and Son?
Was it so, that some sound, as it were a temporal, fleeting creature, had to issue from the mouth of the Father, and strike upon the ear of the Son? Has God a body, that this should proceed, as it were, from His lips? And has the Word the ears of a body, into which sound may come? Lay aside all notions of corporeal forms, regard simplicity, if you are single-minded. But how will you be single-minded? If you will not entangle yourself with the world, but disentangle yourself from the world. For by disentangling yourself, you will be single-minded. And see, if you can, what I say; or if you can not, believe what you do not see. You speak to your son; you speak by a word: neither are you, nor is your son, the word that sounds.
Source: Tractates on the Gospel of John (New Advent)