成唯識論述記 9.414

9.414

述曰：三、八識分別因破外執。第八識名異熟識，何以不俱？彼異熟識是微細劣弱故，此法執望彼麁而強故，此是能熏故彼非也。安惠等師執三性心皆有法執，此識唯異熟故，唯異熟性中破之。

The Commentary states: Third, differentiating it among the eight consciousnesses to refute outside tenets. The eighth consciousness is named the vipāka consciousness; why is it not associated [with the knowledge obstacle]? Because that vipāka consciousness is subtle and weak, whereas this dharmagrāha is coarse and strong in comparison to it; and because this [dharmagrāha] is a perfuming agent, while that [eighth consciousness] is not. Masters such as Sthiramati maintain that the mind of all three natures possesses dharmagrāha; since this consciousness is purely vipāka, it is refuted specifically within its vipāka nature.