成唯識論述記 5.800

5.800

述曰：第二因云：五識生時，必由意識導引五識，方五俱生，亦復同境，方成善、染。先顯五識三性所由，後申俱難，即彼《大論》第三卷，說五識善染必意導生。

The Commentary states: The second reason says: When the five consciousnesses arise, the mental consciousness must guide the five consciousnesses, only after which the five arise together [with it], and also having the same object, only then becoming wholesome or defiled. It first reveals the cause of the three natures of the five consciousnesses, and subsequently presents the objection concerning co-arising; that is, the third fascicle of the Great Treatise says that the wholesome and defiled states of the five consciousnesses must be guided and produced by the mind.