Summa contra Gentiles1.55 Chapter LV. That God understands all things at the same instant

From the foregoing it is also made evident that God understands all things at the same instant.

For our intellect is unable actually to understand several things simultaneously, because since the intellect in act is the thing understood in act, if it were to understand actually several things at the same time, it would follow that the intellect is simultaneously several things according to one genus; which is impossible. And I say according to one genus, because nothing hinders the same subject receiving different forms of different genera, even as the one body receives shape and colour. Now the intelligible species by which the intellect is informed with the result that the things themselves are actually understood are all of one genus: for they have one essential nature although the things whereof they are the species do not agree in one essential nature: wherefore neither are they contrary to one another as are the things outside the mind. Hence it is that, when we consider a certain number of things in any way united together, we understand them at the same time: for we understand a continuous whole simultaneously, and not part by part: and in like manner we understand a proposition, and not the subject first and the predicate afterwards; because we know all the parts by one species of the whole. From this we may gather that whatever number of things are known by one species, they can be understood simultaneously. Now all that God knows, He knows by one species which is His essence. Therefore He can understand all things simultaneously.

Again. The cognitive power does not know a thing except the intention be there, wherefore at times we do not actually imagine the phantasms preserved in the organ, because the intention is not directed thereto: for the appetite moves the other powers to act, in voluntary agents. Hence we do not consider simultaneously a number of things if the intention be not directed to them simultaneously: and those things that must needs come under one intention must be understood simultaneously: since he who considers the comparison between two things, directs his intention simultaneously to both, and considers both at the same time. Now all those things that are in the divine knowledge must come under one intention. For God intends to see His essence perfectly: and this is to see it according to its whole power under which all things are comprised. Therefore God, in seeing His essence, sees all things simultaneously.

Moreover. The intellect of one who considers many things in succession cannot possibly have only one operation: for since operations differ according to their objects, the operation whereby the intellect considers the first thing must needs be distinct from that whereby it considers the second. But the divine intellect has only one operation, which is its essence, as proved above. Therefore it considers all that it knows, not simultaneously but successively.

Further. Succession is inconceivable apart from time, and time apart from movement: since time is the measure of movement according to before or after. Now no movement is possible in God, as may be gathered from what has been said above. Therefore in God's thought there is no succession: and consequently whatever He knows He considers simultaneously.

Again. God's act of understanding is His very being, as shown above. Now there is no before and after in the divine being, but it is all simultaneously, as proved above. Therefore neither is there before and after in God's thought, but He understands all things simultaneously.

Moreover. Every intellect that understands one thing after another is at one time understanding potentially, and at another time actually: for while it understands the first thing actually, it understands the second potentially. But the divine intellect is never in potentiality, but is always understanding actually. Therefore it understands things, not successively, but altogether simultaneously.

Holy Writ bears witness to this truth: for it is said (James i. 17) that with God there is no change, nor shadow of alteration.