Summa contra Gentiles1.48 Chapter XLVIII. That God knows only himself first and per se

From the foregoing it follows that God first and per se knows Himself alone.

For that thing alone is known first and per se by whose species the intellect understands, because the operation is proportionate to the form which is the principle of the operation. Now that by which God understands is nothing else than His essence, as we have proved. Therefore that which is understood by Him first and per se is nothing else than Himself.

Again. It is impossible to understand simultaneously several things first and per se: since one operation cannot terminate simultaneously in several things. Now God understands Himself sometimes, as we have proved. Therefore if He understands something else by way of an object understood first and per se, it follows that His intellect is changed from consideration to consideration of that thing. But this thing is less excellent than He. Therefore the divine intellect would be changed for the worse: which is impossible.

Moreover. The operations of the intellect are distinguished in relation to their objects. If, therefore, God understands Himself and something other than Himself as principal object, He will have several intellectual operations. Therefore either His essence will be divided into 1 Ch. xlvi.. a Ch. xlvii.

several parts, or He will have an intellectual operation that is not His substance: both of which have been proved to be impossible. Therefore it follows that nothing is known by God as understood first and per se, except His essence.

Again. The intellect, in so far as it is distinct from the object of its intelligence, is in potentiality in its regard. If then something else is understood by God first and per se, it will follow that He is in potentiality in respect of something else: and this is impossible as we have shown above.

Further. The thing understood is the perfection of the one who understands: because the intellect is perfect in so far as it actually understands; and this is through its being one with the thing understood. Therefore if something other than God be first understood by Him, something else will be His perfection and more excellent than He. But this is impossible.

Moreover. The knowledge of one who understands is the product of many things understood. Accordingly if many things are known by God as known principally and per se, it follows that God's knowledge is composed of many: and thus either God's essence will be composite, or knowledge will be accidental to God. But either of these is clearly impossible from what has been said. It remains, therefore, that that which is understood by God first and per se is nothing else than His substance.

Further. The intellectual operation takes its species and excellence from that which is understood first and per se; since this is its object. If therefore God understood a thing other than Himself, as though it were understood first and per se, His intellectual operation would derive its species and excellence from that which is other than Himself. But this is impossible: since His operation is His essence, as we have shown. It is accordingly impossible for that which God understands first and per se to be other than Himself.