17 As far as I can, I shall explain my meaning to you, beloved. A carpenter makes a box. First he has the box in design; for if he had it not in design, how could he produce it by workmanship? But the box in theory is not the very box as it appears to the eyes. It exists invisibly in design, it will be visible in the work. Behold, it is made in the work; has it ceased to exist in design? The one is made in the work, and the other remains which exists in design; for that box may rot, and another be fashioned according to that which exists in design.
Give heed, then, to the box as it is in design, and the box as it is in fact. The actual box is not life, the box in design is life; because the soul of the artificer, where all these things are before they are brought forth, is living. So, dearly beloved brethren, because the Wisdom of God, by which all things have been made, contains everything according to design before it is made, therefore those things which are made through this design itself are not immediately life, but whatever has been made is life in Him.
You see the earth, there is an earth in design; you see the sky, there is a sky in design; you see the sun and the moon, these also exist in design: but externally they are bodies, in design they are life. Understand, if in any way you are able, for a great matter has been spoken. If I am not great by whom it is spoken, or through whom it is spoken, still it is from a great authority. For these things are not spoken by me who am small; He is not small to whom I refer in saying these things.
Let each one take in what he can, and to what extent he can; and he who is not able to take in any of it, let him nourish his heart, that he may become able. How is he to nourish it? Let him nourish it with milk, that he may come to strong meat. Let him not leave Christ born through the flesh till he arrive at Christ born of the Father alone, the God-Word with God, through whom all things were made; for that is life, which in Him is the light of men.
Source: Tractates on the Gospel of John (New Advent)