大乘起信論義記 2.132

2.132

疑云：佛得無念，眾生有念。有無懸隔，云何能知也？釋云：眾生有念，本來無念。佛既得彼無念，無念與念本來平等，故云以無念等故，是故得知也。又釋云：以四相念中各即無念故，云以無念等也，是故得無念者，遍知四相念也。

A doubt asks: The Buddha has attained thoughtlessness, while sentient beings have thoughts. Existence and non-existence are far apart; how can he know them? The explanation says: Sentient beings have thoughts, yet originally they are thoughtless. Since the Buddha has attained that thoughtlessness, and thoughtlessness and thought are originally equal, it is said 'because of the equality of thoughtlessness'; therefore he is able to know them. Another explanation says: Because within the thoughts of the four marks, each is itself thoughtlessness, it is said 'because of the equality of thoughtlessness'; therefore the one who attains thoughtlessness comprehensively knows the thoughts of the four marks.