Summa contra Gentiles2.101 Chapter CI. Whether separate substances know all things at the same time by their natural knowledge

Now since the intellect in act is the thing actually understood, as the sense in act is the thing actually sensed; and since the same thing cannot be several things actually at the same time, it would seem impossible for the intellect of a separate substance to have various species of things intelligible, as we have stated above.

But it must be noted that not all is actually understood, the intelligible species of which is actually in the intellect. For since a substance which has understanding has also a will, and consequently has the control of its action, it is in its power, when it already has an intelligible species, to make use of it for understanding actually; or, if it have several species, to use one of them. Wherefore we do not actually consider all the things whereof we possess knowledge. Therefore an intellectual substance that has knowledge through several species, uses one of them as it will, and thereby knows actually at the same time all that he knows by one species; for they are all as one intelligible thing in so far as they are known through one (species), even as our intellect knows at the same time several things compared or related to one another as one individual thing. But it does not know at the same time the things which it knows through different species. Therefore as there is one understanding so is there one thing actually understood.

Accordingly in the intellect of a separate substance there is a certain succession of understandings: there is not, however, movement properly speaking, since act does not succeed potentiality, but act succeeds act. Whereas the divine intellect, forasmuch as it knows all things through one, namely the divine essence, and because its act is its essence, knows all things at the same time. Consequently there is no succession in His understanding, but His act of intelligence is wholly perfect at the same time, and endureth through all ages. Amen.