Objection 1. It seems that God does not know things other than Himself by proper knowledge. For, as was shown (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 (12/2). 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" (Hebrews 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 (Question 4, Article 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 (Question 4, Article 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.
Reply to Objection 1. So to know a thing as it is in the knower, may be understood in two ways. In one way this adverb "so" imports the mode of knowledge on the part of the thing known; and in that sense it is false. For the knower does not always know the object known according to the existence it has in the knower; since the eye does not know a stone according to the existence it has in the eye; but by the image of the stone which is in the eye, the eye knows the stone according to its existence outside the eye. And if any knower has a knowledge of the object known according to the (mode of) existence it has in the knower, the knower nevertheless knows it according to its (mode of) existence outside the knower; thus the intellect knows a stone according to the intelligible existence it has in the intellect, inasmuch as it knows that it understands; while nevertheless it knows what a stone is in its own nature. If however the adverb 'so' be understood to import the mode (of knowledge) on the part of the knower, in that sense it is true that only the knower has knowledge of the object known as it is in the knower; for the more perfectly the thing known is in the knower, the more perfect is the mode of knowledge.
We must say therefore that God not only knows that all things are in Himself; but by the fact that they are in Him, He knows them in their own nature and all the more perfectly, the more perfectly each one is in Him.
Reply to Objection 2. The created essence is compared to the essence of God as the imperfect to the perfect act. Therefore the created essence cannot sufficiently lead us to the knowledge of the divine essence, but rather the converse.
Reply to Objection 3. The same thing cannot be taken in an equal manner as the ratio of different things. But the divine essence excels all creatures. Hence it can be taken as the proper ration of each thing according to the diverse ways in which diverse creatures participate in, and imitate it.
Source: Summa Theologica (New Advent)