Thomas Aquinas, Compendium of Theology

These objections are easily answered if we but penetrate to the truth of the matter. God knows Himself perfectly, and therefore He must have knowledge of all that exists in Himself in any manner whatever. Since every essence and power of created being is from Him, and since whatever comes from anyone exists virtually in him, we necessarily conclude that in knowing Himself He knows the essence of created being and whatever is virtually contained in the latter. And thus He knows all particulars that are virtually in Himself and in all His other causes.

The knowledge possessed by the divine intellect is not like our knowledge, as the first objection urged. Our intellect derives knowledge of things through the species it abstracts, and these are the likenesses of forms and not of matter or of material dispositions, which are principles of individuation. Therefore our intellect cannot know particulars, but only universals. But the divine intellect knows things through its own essence, in which, as in the first principle of being, is virtually contained not only form, but matter. And so God knows not only universals but also particulars.

Likewise God is able to know an infinite number of objects, even though our intellect cannot know the infinite. Our intellect cannot actually contemplate many things at the same time. Hence, if it knew an infinite number of objects, it would have to review them one after another as it contemplated them, which is contrary to the very notion of infinity. However, our intellect can know infinity virtually and potentially, for example, it can know all the species of numbers or of proportions, seeing that it possesses an adequate principle for knowing all things. But God can know many things simultaneously, as was indicated above; and that whereby He knows all things, namely, His essence, is an adequate principle for knowing, not only all that is, but all that can be. Therefore, as our intellect potentially and virtually knows those infinite objects for which it has a principle of cognition, so God actually contemplates all infinities.

Furthermore, although corporeal and temporal particulars do not exist simultaneously, God surely has simultaneous knowledge of them. For He knows them according to His manner of being, which is eternal and without succession. Consequently, as He knows material things in an immaterial way, and many things in unity, so in a single glance He beholds objects that do not exist at the same time. And so His knowledge of particulars does not involve the consequence that anything is added to, or subtracted from, His cognition.

This also makes it clear that He has certain knowledge of contingent things. Even before they come into being, He sees them as they actually exist, and not merely as they will be in the future and as virtually present in their causes, in the way we are able to know some future things. Contingent things, regarded as virtually present in their causes with a claim to future existence, are not sufficiently determinate to admit of certain knowledge about them; but, regarded as actually possessing existence, they are determinate, and hence certain knowledge of them is possible. Thus we can know with the certitude of ocular vision that Socrates is sitting while he is seated. With like certitude God knows, in His eternity, all that takes place throughout the whole course of time. For His eternity is in present contact with the whole course of time, and even passes beyond time. We may fancy that God knows the flight of time in His eternity, in the way that a person standing on top of a watchtower embraces in a single glance a whole caravan of passing travelers.