金剛經纂要刊定記 6.54

6.54

釋悉知。經徵意云：以何義故，能悉知之？釋意云：彼等諸心皆是妄識，妄識即空故云非心，以即空故真心不滅，故云是名真心。體同故能悉知。

Explaining "fully knows": the sūtra raises the question, meaning: by what reason is he able to fully know them? The explanation means: those various minds are all deluded consciousness; because deluded consciousness is empty, it says "are not mind"; because they are empty, the true mind is not annihilated, hence it says "is called the true mind." Because their substance is identical, he is able to know them fully.