Summa Theologica Suppl, q. 92: Of the Vision of the Divine Essence in Reference to the Blessed* (Three Articles) ]
Therefore we must take the other way, which also certain philosophers held, namely Alexander and Averroes (De Anima iii.). For since in every knowledge some form is required whereby the object is known or seen, this form by which the intellect is perfected so as to see separate substances is neither a quiddity abstracted by the intellect from composite things, as the first opinion maintained, nor an impression left on our intellect by the separate substance, as the second opinion affirmed; but the separate substance itself united to our intellect as its form, so as to be both that which is understood, and that whereby it is understood. And whatever may be the case with other separate substances, we must nevertheless allow this to be our way of seeing God in His essence, because by whatever other form our intellect were informed, it could not be led thereby to the Divine essence. This, however, must not be understood as though the Divine essence were in reality the form of our intellect, or as though from its conjunction with our intellect there resulted one being simply, as in natural things from the natural form and matter: but the meaning is that the proportion of the Divine essence to our intellect is as the proportion of form to matter. For whenever two things, one of which is the perfection of the other, are received into the same recipient, the proportion of one to the other, namely of the more perfect to the less perfect, is as the proportion of form to matter: thus light and color are received into a transparent object, light being to color as form to matter. When therefore intellectual light is received into the soul, together with the indwelling Divine essence, though they are not received in the same way, the Divine essence will be to the intellect as form to matter: and that this suffices for the intellect to be able to see the Divine essence by the Divine essence itself may be shown as follows.
As from the natural form (whereby a thing has being) and matter, there results one thing simply, so from the form whereby the intellect understands, and the intellect itself, there results one thing intelligibly. Now in natural things a self-subsistent thing cannot be the form of any matter, if that thing has matter as one of its parts, since it is impossible for matter to be the form of a thing. But if this self-subsistent thing be a mere form, nothing hinders it from being the form of some matter and becoming that whereby the composite itself is [*Literally,—and becoming the 'whereby-it-is' of the composite itself] as instanced in the soul. Now in the intellect we must take the intellect itself in potentiality as matter, and the intelligible species as form; so that the intellect actually understanding will be the composite as it were resulting from both. Hence if there be a self-subsistent thing, that has nothing in itself besides that which is intelligible, such a thing can by itself be the form whereby the intellect understands. Now a thing is intelligible in respect of its actuality and not of its potentiality (Met. ix): in proof of which an intelligible form needs to be abstracted from matter and from all the properties of matter. Therefore, since the Divine essence is pure act, it will be possible for it to be the form whereby the intellect understands: and this will be the beatific vision. Hence the Master says (Sent. ii, D, 1) that the union of the body with the soul is an illustration of the blissful union of the spirit with God.
Reply to Objection 1The words quoted can be explained in three ways, according to Augustine (De Videndo Deo: Ep. cxlvii). In one way as excluding corporeal vision, whereby no one ever saw or will see God in His essence; secondly, as excluding intellectual vision of God in His essence from those who dwell in this mortal flesh; thirdly, as excluding the vision of comprehension from a created intellect. It is thus that Chrysostom understands the saying wherefore he adds: "By seeing, the evangelist means a most clear perception, and such a comprehension as the Father has of the Son." This also is the meaning of the evangelist, since he adds: "The Only-begotten Son Who is in the bosom of the Father, He hath declared Him": his intention being to prove the Son to be God from His comprehending God.
Reply to Objection 2Just as God, by His infinite essence, surpasses all existing things which have a determinate being, so His knowledge, whereby He knows, is above all knowledge. Wherefore as our knowledge is to our created essence, so is the Divine knowledge to His infinite essence. Now two things contribute to knowledge, to wit, the knower and the thing known. Again, the vision whereby we shall see God in His essence is the same whereby God sees Himself, as regards that whereby He is seen, because as He sees Himself in His essence so shall we also see Him. But as regards the knower there is the difference that is between the Divine intellect and ours. Now in the order of knowledge the object known follows the form by which we know, since by the form of a stone we see a stone: whereas the efficacy of knowledge follows the power of the knower: thus he who has stronger sight sees more clearly. Consequently in that vision we shall see the same thing that God sees, namely His essence, but not so effectively.
Reply to Objection 3Dionysius is speaking there of the knowledge whereby wayfarers know God by a created form, whereby our intellect is informed so as to see God. But as Augustine says (De Videndo Deo: Ep. cxlvii), "God evades every form of our intellect," because whatever form our intellect conceive, that form is out of proportion to the Divine essence. Hence He cannot be fathomed by our intellect: but our most perfect knowledge of Him as wayfarers is to know that He is above all that our intellect can conceive, and thus we are united to Him as to something unknown. In heaven, however, we shall see Him by a form which is His essence, and we shall be united to Him as to something known.
Reply to Objection 4God is light (Jn. 1:9). Now illumination is the impression of light on an illuminated object. And since the Divine essence is of a different mode from any likeness thereof impressed on the intellect, he (Dionysius) says that the "Divine darkness is impervious to all illumination," because, to wit, the Divine essence, which he calls "darkness" on account of its surpassing brightness, remains undemonstrated by the impression on our intellect, and consequently is "hidden from all knowledge." Therefore if anyone in seeing God conceives something in his mind, this is not God but one of God's effects.
Reply to Objection 5Although the glory of God surpasses any form by which our intellect is informed now, it does not surpass the Divine essence, which will be the form of our intellect in heaven: and therefore although it is invisible now, it will be visible then.
Reply to Objection 6Although there can be no proportion between finite and infinite, since the excess of the infinite over the finite is indeterminate, there can be proportionateness or a likeness to proportion between them: for as a finite thing is equal to some finite thing, so is an infinite thing equal to an infinite thing. Now in order that a thing be known totally, it is sometimes necessary that there be proportion between knower and known, because the power of the knower needs to be adequate to the knowableness of the thing known, and equality is a kind of proportion. Sometimes, however, the knowableness of the thing surpasses the power of the knower, as when we know God, or conversely when He knows creatures: and then there is no need for proportion between knower and known, but only for proportionateness; so that, to wit, as the knower is to the knowable object, so is the knowable object to the fact of its being known: and this proportionateness suffices for the infinite to be known by the finite, or conversely.