I answer that, It must be said that the act of God's intellect is His substance. For if His act of understanding were other than His substance, then something else, as the Philosopher says (Metaph. xii), would be the act and perfection of the divine substance, to which the divine substance would be related, as potentiality is to act, which is altogether impossible; because the act of understanding is the perfection and act of the one understanding. Let us now consider how this is. As was laid down above (A[2]), to understand is not an act passing to anything extrinsic; for it remains in the operator as his own act and perfection; as existence is the perfection of the one existing: just as existence follows on the form, so in like manner to understand follows on the intelligible species. Now in God there is no form which is something other than His existence, as shown above (Q[3]). Hence as His essence itself is also His intelligible species, it necessarily follows that His act of understanding must be His essence and His existence.
Thus it follows from all the foregoing that in God, intellect, and the object understood, and the intelligible species, and His act of understanding are entirely one and the same. Hence when God is said to be understanding, no kind of multiplicity is attached to His substance.
Reply to Objection 1: To understand is not an operation proceeding out of the operator, but remaining in him.
Reply to Objection 2: When that act of understanding which is not subsistent is understood, something not great is understood; as when we understand our act of understanding; and so this cannot be likened to the act of the divine understanding which is subsistent.
Thus appears the Reply to the Third Objection. For the act of divine understanding subsists in itself, and belongs to its very self and is not another's; hence it need not proceed to infinity.
Whether God knows things other than Himself?
Objection 1: It seems that God does not know things besides Himself. For all other things but God are outside of God. But Augustine says (Octog. Tri. Quaest. qu. xlvi) that "God does not behold anything out of Himself." Therefore He does not know things other than Himself.
Objection 2: Further, the object understood is the perfection of the one who understands. If therefore God understands other things besides Himself, something else will be the perfection of God, and will be nobler than He; which is impossible.
Objection 3: Further, the act of understanding is specified by the intelligible object, as is every other act from its own object. Hence the intellectual act is so much the nobler, the nobler the object understood. But God is His own intellectual act. If therefore God understands anything other than Himself, then God Himself is specified by something else than Himself; which cannot be. Therefore He does not understand things other than Himself.
On the contrary, It is written: "All things are naked and open to His eyes" ( Heb. 4:13 ).
I answer that, God necessarily knows things other than Himself. For it is manifest that He perfectly understands Himself; otherwise His existence would not be perfect, since His existence is His act of understanding. Now if anything is perfectly known, it follows of necessity that its power is perfectly known. But the power of anything can be perfectly known only by knowing to what its power extends. Since therefore the divine power extends to other things by the very fact that it is the first effective cause of all things, as is clear from the aforesaid (Q[2], A[3]), God must necessarily know things other than Himself. And this appears still more plainly if we add that the every existence of the first effective cause---viz. God---is His own act of understanding. Hence whatever effects pre-exist in God, as in the first cause, must be in His act of understanding, and all things must be in Him according to an intelligible mode: for everything which is in another, is in it according to the mode of that in which it is.
Now in order to know how God knows things other than Himself, we must consider that a thing is known in two ways: in itself, and in another. A thing is known in itself when it is known by the proper species adequate to the knowable object; as when the eye sees a man through the image of a man. A thing is seen in another through the image of that which contains it; as when a part is seen in the whole by the image of the whole; or when a man is seen in a mirror by the image in the mirror, or by any other mode by which one thing is seen in another.
So we say that God sees Himself in Himself, because He sees Himself through His essence; and He sees other things not in themselves, but in Himself; inasmuch as His essence contains the similitude of things other than Himself.
Reply to Objection 1: The passage of Augustine in which it is said that God "sees nothing outside Himself" is not to be taken in such a way, as if God saw nothing outside Himself, but in the sense that what is outside Himself He does not see except in Himself, as above explained.
Reply to Objection 2: The object understood is a perfection of the one understanding not by its substance, but by its image, according to which it is in the intellect, as its form and perfection, as is said in De Anima iii. For "a stone is not in the soul, but its image." Now those things which are other than God are understood by God, inasmuch as the essence of God contains their images as above explained; hence it does not follow that there is any perfection in the divine intellect other than the divine essence.
Reply to Objection 3: The intellectual act is not specified by what is understood in another, but by the principal object understood in which other things are understood. For the intellectual act is specified by its object, inasmuch as the intelligible form is the principle of the intellectual operation: since every operation is specified by the form which is its principle of operation; as heating by heat. Hence the intellectual operation is specified by that intelligible form which makes the intellect in act. And this is the image of the principal thing understood, which in God is nothing but His own essence in which all images of things are comprehended. Hence it does not follow that the divine intellectual act, or rather God Himself, is specified by anything else than the divine essence itself.
Whether God knows things other than Himself by proper knowledge?
Objection 1: It seems that God does not know things other than Himself by proper knowledge. For, as was shown (A[5]), God knows things other than Himself, according as they are in Himself. But other things are in Him as in their common and universal cause, and are known by God as in their first and universal cause. This is to know them by general, and not by proper knowledge. Therefore God knows things besides Himself by general, and not by proper knowledge.
Objection 2: Further, the created essence is as distant from the divine essence, as the divine essence is distant from the created essence. But the divine essence cannot be known by the created essence, as said above (Q[12]/A[2]). Therefore neither can the created essence be known by the divine essence. Thus as God knows only by His essence, it follows that He does not know what the creature is in its essence, so as to know "what it is," which is to have proper knowledge of it.
Objection 3: Further, proper knowledge of a thing can come only through its proper ratio. But as God knows all things by His essence, it seems that He does not know each thing by its proper ratio; for one thing cannot be the proper ratio of many and diverse things. Therefore God has not a proper knowledge of things, but a general knowledge; for to know things otherwise than by their proper ratio is to have only a common and general knowledge of them.
On the contrary, To have a proper knowledge of things is to know them not only in general, but as they are distinct from each other. Now God knows things in that manner. Hence it is written that He reaches "even to the division of the soul and the spirit, of the joints also and the marrow, and is a discerner of thoughts and intents of the heart; neither is there any creature invisible in His sight" ( Heb. 4:12 , 13 ).
I answer that, Some have erred on this point, saying that God knows things other than Himself only in general, that is, only as beings. For as fire, if it knew the nature of heat, and all things else in so far as they are hot; so God, through knowing Himself as the principle of being, knows the nature of being, and all other things in so far as they are beings.
But this cannot be. For to know a thing in general and not in particular, is to have an imperfect knowledge. Hence our intellect, when it is reduced from potentiality to act, acquires first a universal and confused knowledge of things, before it knows them in particular; as proceeding from the imperfect to the perfect, as is clear from Phys. i. If therefore the knowledge of God regarding things other than Himself is only universal and not special, it would follow that His understanding would not be absolutely perfect; therefore neither would His being be perfect; and this is against what was said above (Q[4], A[1]). We must therefore hold that God knows things other than Himself with a proper knowledge; not only in so far as being is common to them, but in so far as one is distinguished from the other. In proof thereof we may observe that some wishing to show that God knows many things by one, bring forward some examples, as, for instance, that if the centre knew itself, it would know all lines that proceed from the centre; or if light knew itself, it would know all colors.
Now these examples although they are similar in part, namely, as regards universal causality, nevertheless they fail in this respect, that multitude and diversity are caused by the one universal principle, not as regards that which is the principle of distinction, but only as regards that in which they communicate. For the diversity of colors is not caused by the light only, but by the different disposition of the diaphanous medium which receives it; and likewise, the diversity of the lines is caused by their different position. Hence it is that this kind of diversity and multitude cannot be known in its principle by proper knowledge, but only in a general way. In God, however, it is otherwise. For it was shown above (Q[4], A[2]) that whatever perfection exists in any creature, wholly pre-exists and is contained in God in an excelling manner. Now not only what is common to creatures--viz. being---belongs to their perfection, but also what makes them distinguished from each other; as living and understanding, and the like, whereby living beings are distinguished from the non-living, and the intelligent from the non-intelligent. Likewise every form whereby each thing is constituted in its own species, is a perfection; and thus all things pre-exist in God, not only as regards what is common to all, but also as regards what distinguishes one thing from another. And therefore as God contains all perfections in Himself, the essence of God is compared to all other essences of things, not as the common to the proper, as unity is to numbers, or as the centre (of a circle) to the (radiating) lines; but as perfect acts to imperfect; as if I were to compare man to animal; or six, a perfect number, to the imperfect numbers contained under it. Now it is manifest that by a perfect act imperfect acts can be known not only in general, but also by proper knowledge; thus, for example, whoever knows a man, knows an animal by proper knowledge; and whoever knows the number six, knows the number three also by proper knowledge.
As therefore the essence of God contains in itself all the perfection contained in the essence of any other being, and far more, God can know in Himself all of them with proper knowledge. For the nature proper to each thing consists in some degree of participation in the divine perfection. Now God could not be said to know Himself perfectly unless He knew all the ways in which His own perfection can be shared by others. Neither could He know the very nature of being perfectly, unless He knew all modes of being. Hence it is manifest that God knows all things with proper knowledge, in their distinction from each other.