That God primarily and essentially knows Himself alone

THE Understanding is in potentiality in regard of its object, in so far as it is a different thing from that object. If therefore the primary and essential object of divine understanding be something different from God, it will follow that God is in potentiality in respect of some other thing, which is impossible (Chap. XVI).

5. A thing understood is the perfection of him who understands it: for an understanding is perfected by actually understanding, which means being made one with the object understood.* If therefore anything else than God is the first object of His understanding, something else will be His perfection, and will be nobler than He, which is impossible.

1.47 : That God perfectly understands Himself
1.49 : That God knows other things besides Himself