大乘起信論義記 3.23

3.23

This also explains the reason why the above avidyā is able to arise. Because of not understanding the equal, unified meaning of tathatā, the mind is unconjoined, and suddenly a thought stirs, which is named avidyā. This reveals that fundamental avidyā is extremely subtle; there is not yet the distinction of subject and object or mental king and mental factors. Being a delusion of the mind itself, it is said to be unconjoined, unlike the conjunction of the mental king and mental factors. Only this avidyā is the source of defiled phenomena, being the most extremely subtle; there is no other defiled phenomenon that can serve as its root, hence it is said "suddenly thought arises". As the Yingluo benye jing says: "Before the four abiding stages, no further phenomena arise, hence it is called the beginningless abiding stage of avidyā." This clarifies that before avidyā there is no separate phenomenon that serves as the root of its initial accumulation; hence it is called "beginningless", which is precisely the meaning of "suddenly" in this treatise. This is spoken of as "having nothing prior" and also called "suddenly" in terms of the gateway of the mutual reliance of coarse and subtle, not in terms of time periods, because its arising has no beginning.