Thomas Aquinas, Compendium of Theology

The possibility of such knowledge must be investigated. Manifestly, since our intellect knows nothing except through an intelligible species of the thing known, the species of one thing cannot disclose the essense of another thing. In proportion as the species, whereby the mind knows, is remote from the thing known, the less perfect is the knowledge our intellect has of that thing's essence. For example, if we should know an ox by the species of an ass, we would have an imperfect knowledge of the essence of the ox, for our concept would be limited to its genus. Our knowledge would be still more defective if we were to know the ox through the medium of a stone, because then we would know it by a more remote genus. And if our knowledge were gained through the species of a thing that did not agree with the ox in any genus, we could not know the essence of the ox at all.

Previous discussion has brought out the fact that no creature is associated with God in genus. Hence the essence of God cannot be known through any created species whatever, whether sensible or intelligible. Accordingly, if God is to be known as He is, in His essence, God Himself must become the form of the intellect knowing Him and must be joined to that intellect, not indeed so as to constitute a single nature with it, but in the way an intelligible species is joined to the intelligence. For God, who is His own being, is also His own truth, and truth is the form of the intellect.

Whatever receives a form, must first acquire the disposition requisite to the reception of that form. Our intellect is not equipped by its nature with the ultimate disposition looking to that form which is truth; otherwise it would be in possession of truth from the beginning. Consequently, when it does finally attain to truth, it must be elevated by some disposition newly conferred on it. And this we call the light of glory, whereby our intellect is perfected by God, who alone by His very nature has this form properly as His own. In somewhat the same way the disposition which heat has for the form of fire can come from fire alone. This is the light that is spoken of in Psalm 35:10: "In Thy light we shall see light."