大乘起信論義記 3.183

3.183

略中疑云：既其不二，何以說別？釋云：以依生滅識相恒沙染法，返此表示真如淨德恒沙差別。且舉其染本故，但云業識。

In the brief explanation, an objection is raised: "Since it is non-dual, why speak of differentiation?" The explanation states: Relying on the defiled dharmas [as numerous as] the sands of the Ganges in the marks of the consciousness of arising and ceasing, by contrast with this the differentiation of the pure virtues [as numerous as] the sands of the Ganges in tathatā is demonstrated. And because the root of these defilements is cited first, it speaks merely of the karmic consciousness.