But let us see further what the things are which he proposes to teach us, if indeed we can comprehend them, since he speaks of us as being “utterly wedded to the flesh;” although if we live well, and in accordance with the teaching of Jesus, we hear this said of us: “You are not in the flesh, but in the Spirit, if the Spirit of God dwells in you.” He says also that we look upon nothing that is pure, although our endeavour is to keep even our thoughts free from all defilement of sin, and although in prayer we say, “Create in me a clean heart, O God, and renew a right spirit within me,” so that we may behold Him with that “pure heart” to which alone is granted the privilege of seeing Him.
This, then, is what he proposes for our instruction: “Things are either intelligible, which we call substance— being; or visible, which we call becoming: with the former is truth; from the latter arises error. Truth is the object of knowledge; truth and error form opinion. Intelligible objects are known by the reason, visible objects by the eyes; the action of the reason is called intelligent perception, that of the eyes vision. As, then, among visible things the sun is neither the eye nor vision, but that which enables the eye to see, and renders vision possible, and in consequence of it visible things are seen, all sensible things exist and itself is rendered visible; so among things intelligible, that which is neither reason, nor intelligent perception, nor knowledge, is yet the cause which enables the reason to know, which renders intelligent perception possible; and in consequence of it knowledge arises, all things intelligible, truth itself and substance have their existence; and itself, which is above all these things, becomes in some ineffable way intelligible.
These things are offered to the consideration of the intelligent; and if even you can understand any of them, it is well. And if you think that a Divine Spirit has descended from God to announce divine things to men, it is doubtless this same Spirit that reveals these truths, and it was under the same influence that men of old made known many important truths. But if you cannot comprehend these things, then keep silence; do not expose your own ignorance, and do not accuse of blindness those who see, or of lameness those who run, while you yourselves are utterly lamed and mutilated in mind, and lead a merely animal life— the life of the body, which is the dead part of our nature.”
Source: Against Celsus (New Advent)