肇論新疏 2.89

2.89

所知者，即真諦。恐難者再救云：經稱清淨，非約能知般若無知無見，但約所知真諦清淨，故美般若云清淨者。

"The known" is the ultimate truth. He fears that the objector might attempt another rescue, saying: "The sūtra's calling it pure is not in terms of the knowing prajñā being without knowing and without seeing, but merely in terms of the known ultimate truth being pure; hence prajñā is praised as pure."