I answer that, Even as we hold by faith that the last end of man's life is to see God, so the philosophers maintained that man's ultimate happiness is to understand immaterial substances according to their being. Hence in reference to this question we find that philosophers and theologians encounter the same difficulty and the same difference of opinion. For some philosophers held that our passive intellect can never come to understand separate substances. thus Alfarabius expresses himself at the end of his Ethics, although he says the contrary in his book On the Intelligence, as the Commentator attests (De Anima iii). In like manner certain theologians held that the human intellect can never attain to the vision of God in His essence. on either side they were moved by the distance which separates our intellect from the Divine essence and from separate substances. For since the intellect in act is somewhat one with the intelligible object in act, it would seem difficult to understand how the created intellect is made to be an uncreated essence. Wherefore Chrysostom says (Hom. xiv in Joan.): "How can the creature see the uncreated?" Those who hold the passive intellect to be the subject of generation and corruption, as being a power dependent on the body, encounter a still greater difficulty not only as regards the vision of God but also as regards the vision of any separate substances. But this opinion is altogether untenable. First, because it is in contradiction to the authority of canonical scripture, as Augustine declares (De Videndo Deo: Ep. cxlvii). Secondly, because, since understanding is an operation most proper to man, it follows that his happiness must be held to consist in that operation when perfected in him. Now since the perfection of an intelligent being as such is the intelligible object, if in the most perfect operation of his intellect man does not attain to the vision of the Divine essence, but to something else, we shall be forced to conclude that something other than God is the object of man's happiness: and since the ultimate perfection of a thing consists in its being united to its principle, it follows that something other than God is the effective principle of man, which is absurd, according to us, and also according to the philosophers who maintain that our souls emanate from the separate substances, so that finally we may be able to understand these substances. Consequently, according to us, it must be asserted that our intellect will at length attain to the vision of the Divine essence, and according to the philosophers, that it will attain to the vision of separate substances.
It remains, then, to examine how this may come about. For some, like Alfarabius and Avempace, held that from the very fact that our intellect understands any intelligible objects whatever, it attains to the vision of a separate substance. To prove this they employ two arguments. The first is that just as the specific nature is not diversified in various individuals, except as united to various individuating principles, so the idea understood is not diversified in me and you, except in so far as it is united to various imaginary forms: and consequently when the intellect separates the idea understood from the imaginary forms, there remains a quiddity understood, which is one and the same in the various persons understanding it, and such is the quiddity of a separate substance. Hence, when our intellect attains to the supreme abstraction of any intelligible quiddity, it thereby understands the quiddity of the separate substance that is similar to it. The second argument is that our intellect has a natural aptitude to abstract the quiddity from all intelligible objects having a quiddity. If, then, the quiddity which it abstracts from some particular individual be a quiddity without a quiddity, the intellect by understanding it understands the quiddity of the separate substance which has a like disposition, since separate substances are subsisting quiddities without quiddities; for the quiddity of a simple thing is the simple thing itself, as Avicenna says (Met. iii). On the other hand if the quiddity abstracted from this particular sensible be a quiddity that has a quiddity, it follows that the intellect has a natural aptitude to abstract this quiddity, and consequently since we cannot go on indefinitely, we shall come to some quiddity without a quiddity, and this is what we understand by a separate quiddity [*Cf. FP, Question [88], Article [2]].
But this reasoning is seemingly inconclusive. First, because the quiddity of the material substance, which the intellect abstracts, is not of the same nature as the quiddity of the separate substances, and consequently from the fact that our intellect abstracts the quiddities of material substances and knows them, it does not follow that it knows the quiddity of a separate substance, especially of the Divine essence, which more than any other is of a different nature from any created quiddity. Secondly, because granted that it be of the same nature, nevertheless the knowledge of a composite thing would not lead to the knowledge of a separate substance, except in the point of the most remote genus, namely substance: and such a knowledge is imperfect unless it reach to the properties of a thing. For to know a man only as an animal is to know him only in a restricted sense and potentially: and much less is it to know only the nature of substance in him. Hence to know God thus, or other separate substances, is not to see the essence of God or the quiddity of a separate substance, but to know Him in His effect and in a mirror as it were. For this reason Avicenna in his Metaphysics. propounds another way of understanding separate substances, to wit that separate substances are understood by us by means of intentions of their quiddities, such intentions being images of their substances, not indeed abstracted therefrom, since they are immaterial, but impressed thereby on our souls. But this way also seems inadequate to the Divine vision which we seek. For it is agreed that "whatever is received into any thing is therein after the mode of the recipient": and consequently the likeness of the Divine essence impressed on our intellect will be according to the mode of our intellect: and the mode of our intellect falls short of a perfect reception of the Divine likeness. Now the lack of perfect likeness may occur in as many ways, as unlikeness may occur. For in one way there is a deficient likeness, when the form is participated according to the same specific nature, but not in the same measure of perfection: such is the defective likeness in a subject that has little whiteness in comparison with one that has much. In another way the likeness is yet more defective, when it does not attain to the same specific nature but only to the same generic nature: such is the likeness of an orange-colored or yellowish object in comparison with a white one. In another way, still more defective is the likeness when it does not attain to the same generic nature, but only to a certain analogy or proportion: such is the likeness of whiteness to man, in that each is a being: and in this way every likeness received into a creature is defective in comparison with the Divine essence. Now in order that the sight know whiteness, it is necessary for it to receive the likeness of whiteness according to its specific nature, although not according to the same manner of being because the form has a manner of being in the sense other from that which it has in the thing outside the soul: for if the form of yellowness were received into the eye, the eye would not be said to see whiteness. In like manner in order that the intellect understand a quiddity, it is necessary for it to receive its likeness according to the same specific nature, although there may possibly not be the same manner of being on either side: for the form which is in the intellect or sense is not the principle of knowledge according to its manner of being on both sides, but according to its common ratio with the external object. Hence it is clear that by no likeness received in the created intellect can God be understood, so that His essence be seen immediately. And for this reason those who held the Divine essence to be seen in this way alone, said that the essence itself will not be seen, but a certain brightness, as it were a radiance thereof. Consequently neither does this way suffice for the Divine vision that we seek.
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.