Since therefore God is in the highest degree of immateriality it follows that He occupies the highest place in knowledge. Knowledge is not a quality of God, nor a habit; but substance and pure act. Knowledge is according to the mode of the one who knows; for the thing known is in the knower according to the mode of the knower.
For God, this means He understands Himself through Himself. In proof whereof it must be known that although in operations which pass to an external effect, the object of the operation, which is taken as the term, exists outside the operator; nevertheless in operations that remain in the operator, the object signified as the term of operation, resides in the operator; and accordingly as it is in the operator, the operation is actual. Since therefore God has nothing in Him of potentiality, but is pure act, His intellect and its object are altogether the same.
Our passive intellect can be exercised concerning intelligible objects only so far as it is perfected by the intelligible species of something; and in that way it understands itself by an intelligible species, as it understands other things. But God is a pure act in the order of existence, as also in the order of intelligible objects; therefore He understands Himself through Himself.
Can God comprehend Himself? “Whatever comprehends itself is finite as regards itself,” but God is in all ways infinite.
Indeed, God perfectly comprehends Himself. A thing is said to be comprehended when the end of the knowledge of it is attained, and this is accomplished when it is known as perfectly as it is knowable. God knows Himself as perfectly as He is perfectly knowable. For everything is knowable according to the mode of its own actuality. (Do we see a recurring theme here?)
The act of God’s intellect is His substance. God is Who is. We attribute various perfections and acts to God from our viewpoint, but in God, they are simply one, pure act. As Augustine says “In God to be is the same as to be wise.” God’s existence is His substance. In God, intellect, and the object understood, and the intelligible species, and His act of understanding are entirely one and the same.
God necessarily knows things other than Himself, indeed all things, and these precisely through knowing Himself, Who is the cause of all things.
Whatever effects pre-exist in God, as in the first cause, must be in His act of understanding. Everything outside of God is in fact an effect of God, for He is the Creator.
In Aristotle’s De Anima it is said that “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. For any that fear a sort of pantheism here, this fear is unfounded. The things known in God are not part of God.
God’s universal knowledge of all things through Himself does not mean He does not know particulars, for 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). This differs from Avicenna, the Arabic philosopher who would say God only knows creatures in a general way, but not particulars.
We go discursively from causes to things caused, but in God this is not so, for He knows all simply that it is. As Augustine “God does not see all things in their particularity or separately, as if He saw alternately here and there; but He sees all things together at once.” Altogether there is only one act of understanding in itself, nevertheless many things may be understood in one medium.
God’s knowledge is itself the cause of things. Again quoting St. Augustine, “Not because they are, does God know all creatures spiritual and temporal, but because He knows them, therefore they are.” And Thomas says “For the knowledge of God is to all creatures what the knowledge of the artificer is to things made by his art.”
God knows all things whatsoever that in any way are, including if they are not. Indeed, He knows all possible things, even things that will never be. Of course, He does not know them as existing, for they do not and never will exist, and to know things that are not as if they were would be to not know them properly. It is not necessary that everything that God knows actually come to be, for this would be a philosophy of deterministic creation, rather than God’s freely creating whatever He wills.
This is then the proper place to ask if God knows evil things, for evil, properly understood, is what is not in where it should be. “Evil is the privation of good.” God’s knowledge is the cause of what is, but not of what is not, and so is not the cause of evil, although He knows what should be where it is not. In Thomas’ words, “The knowledge of God is not the cause of evil; but is the cause of the good whereby evil is known.”
Further, God can and does know infinite things, future contingent things, even speculatively, and all things as they are in any way knowable, as long as this is in a way of perfection. Just as God is wise without any of the lack of perfection we attribute to wise men and good without any of the imperfections that go along with goodness in creation, He knows, without any of the imperfections that must be found in the knowledge of created beings.
In the end, it all comes down to the simplicity of God, which for us is the most complex all of possible things, and why we will never comprehend, even in the beatific vision, this perfect simplicity, this all knowing, God who simply Is.