Summa Theologica Suppl, q. 92: Of the Vision of the Divine Essence in Reference to the Blessed* (Three Articles) ]
We may also reply that proportion according to the strict sense in which it is employed signifies a ratio of quantity to quantity based on a certain fixed excess or equality; but is further transferred to denote any ratio of any one thing to another; and in this sense we say that matter should be proportionate to its form. In this sense nothing hinders our intellect, although finite, being described as proportionate to the vision of the Divine essence; but not to the comprehension thereof, on account of its immensity.
Reply to Objection 7Likeness and distance are twofold. One is according to agreement in nature; and thus God is more distant from the created intellect than the created intelligible is from the sense. The other is according to proportionateness; and thus it is the other way about, for sense is not proportionate to the knowledge of the immaterial, as the intellect is proportionate to the knowledge of any immaterial object whatsoever. It is this likeness and not the former that is requisite for knowledge, for it is clear that the intellect understanding a stone is not like it in its natural being; thus also the sight apprehends red honey and red gall, though it does not apprehend sweet honey, for the redness of gall is more becoming to honey as visible, than the sweetness of honey to honey.
Reply to Objection 8In the vision wherein God will be seen in His essence, the Divine essence itself will be the form, as it were, of the intellect, by which it will understand: nor is it necessary for them to become one in being, but only to become one as regards the act of understanding.
Reply to Objection 9We do not uphold the saying of Avicenna as regards the point at issue, for in this other philosophers also disagree with him. Unless perhaps we might say that Avicenna refers to the knowledge of separate substances, in so far as they are known by the habits of speculative sciences and the likeness of other things. Hence he makes this statement in order to prove that in us knowledge is not a substance but an accident. Nevertheless, although the Divine essence is more distant, as to the property of its nature, from our intellect, than is the substance of an angel, it surpasses it in the point of intelligibility, since it is pure act without any admixture of potentiality, which is not the case with other separate substances. Nor will that knowledge whereby we shall see God in His essence be in the genus of accident as regards that whereby He will be seen, but only as regards the act of the one who understands Him, for this act will not be the very substance either of the person understanding or of the thing understood.
Reply to Objection 10A substance that is separate from matter understands both itself and other things; and in both cases the authority quoted can be verified. For since the very essence of a separate substance is of itself intelligible and actual, through being separate from matter, it is clear that when a separate substance understands itself, that which understands and that which is understood are absolutely identical, for it does not understand itself by an intention abstracted from itself, as we understand material objects. And this is apparently the meaning of the Philosopher (De Anima iii.) as indicated by the Commentator (De Anima iii). But when it understands other things, the object actually understood becomes one with the intellect in act, in so far as the form of the object understood becomes the form of the intellect, for as much as the intellect is in act; not that it becomes identified with the essence of the intellect, as Avicenna proves (De Natural. vi.), because the essence of the intellect remains one under two forms whereby it understands two things in succession, in the same way as primary matter remains one under various forms. Hence also the Commentator (De Anima iii.) compares the passive intellect, in this respect, to primary matter. Thus it by no means follows that our intellect in seeing God becomes the very essence of God, but that the latter is compared to it as its perfection or form.
Reply to Objection 11These and all like authorities must be understood to refer to the knowledge whereby we know God on the way, for the reason given above.
Reply to Objection 12The infinite is unknown if we take it in the privative sense, as such, because it indicates removal of completion whence knowledge of a thing is derived. Wherefore the infinite amounts to the same as matter subject to privation, as stated in Phys. iii. But if we take the infinite in the negative sense, it indicates the absence of limiting matter, since even a form is somewhat limited by its matter. Hence the infinite in this sense is of itself most knowable; and it is in this way that God is infinite.
Reply to Objection 13Augustine is speaking of bodily vision, by which God will never be seen. This is evident from what precedes: "For no man hath seen God at any time, nor can any man see Him as these things which we call visible are seen: in this way He is by nature invisible even as He is incorruptible." As, however, He is by nature supremely being, so He is in Himself supremely intelligible. But that He be for a time not understood by us is owing to our defect: wherefore that He be seen by us after being unseen is owing to a change not in Him but in us.
Reply to Objection 14In heaven God will be seen by the saints as He is, if this be referred to the mode of the object seen, for the saints will see that God has the mode which He has. But if we refer the mode to the knower, He will not be seen as He is, because the created intellect will not have so great an efficacy in seeing, as the Divine essence has to the effect of being seen.
Reply to Objection 15There is a threefold medium both in bodily and in intellectual vision. The first is the medium "under which" the object is seen, and this is something perfecting the sight so as to see in general, without determining the sight to any particular object. Such is bodily light in relation to bodily vision; and the light of the active intellect in relation to the passive intellect, in so far as this light is a medium. The second is the light "by which" the object is seen, and this is the visible form whereby either sight is determined to a special object, for instance by the form of a stone to know a stone. The third is the medium "in which" it is seen; and this is something by gazing on which the sight is led to something else: thus by looking in a mirror it is led to see the things reflected in the mirror, and by looking at an image it is led to the thing represented by the image. In this way, too, the intellect from knowing an effect is led to the cause, or conversely. Accordingly in the heavenly vision there will be no third medium, so that, to wit, God be known by the images of other things, as He is known now, for which reason we are said to see now in a glass: nor will there be the second medium, because the essence itself of God will be that whereby our intellect will see God. But there will only be the first medium, which will upraise our intellect so that it will be possible for it to be united to the uncreated substance in the aforesaid manner. Yet this medium will not cause that knowledge to be mediate, because it does not come in between the knower and the thing known, but is that which gives the knower the power to know [*Cf. FP, Q[12], A[5]].
Reply to Objection 16Corporeal creatures are not said to be seen immediately, except when that which in them is capable of being brought into conjunction with the sight is in conjunction therewith. Now they are not capable of being in conjunction with the sight of their essence on account of their materiality: hence they are seen immediately when their image is in conjunction with the sight. But God is able to be united to the intellect by His essence: wherefore He would not be seen immediately, unless His essence were united to the intellect: and this vision, which is effected immediately, is called "vision of face." Moreover the likeness of the corporeal object is received into the sight according to the same ratio as it is in the object, although not according to the same mode of being. Wherefore this likeness leads to the object directly: whereas no likeness can lead our intellect in this way to God, as shown above: and for this reason the comparison fails.