大乘起信論廣釋卷第三．四．五 3.59

3.59

論。四者名為至染淨法故　釋曰。是事識中細分法執不了前心所現境空而起染淨勝劣分別。故言分別染淨法也。

The treatise says: "Fourth is called [the cognitive consciousness], because [it discriminates] defiled and pure dharmas."

Exposition: This is the subtle stage of phenomenal consciousness. Not understanding that the objects manifested by the preceding mind are empty, it generates discriminations of defiled and pure, superior and inferior; hence it says "discriminating defiled and pure dharmas."