大乘起信論義記 2.187

2.187

「又一切染法」下，釋後因義。染法不能染者，以性淨故。雖現染法，非染所污，非直現染之時非染所染，亦乃由現染故反顯本淨。如鏡明淨能現穢物，穢物現時反顯鏡淨，豈此穢物能污鏡耶？若不現染，則無以顯其不染也。智體不動者，以本無染，今無始淨，是故本覺之智未曾移動。又雖現染法，不為所染，故云不動。如鏡中像隨質轉變，然其鏡體未曾動也。具足無漏等者，此本覺中恒沙性德無所少也。又與眾生作內熏之因，令厭生死樂求涅槃，故《勝鬘經》「由有如來藏，能厭生死苦，樂求涅槃也。」《佛性論》云「自性清淨心名為道諦也。」又十種佛性中業性也。

The passage from “Furthermore, it cannot be defiled by any defiled phenomena” onward explains the meaning of the second cause. “It cannot be defiled by defiled phenomena” is because its nature is pure. Although it manifests defiled phenomena, it is not tainted by defilement; not only is it not defiled by defilement at the very time of manifesting defilement, but it is precisely by manifesting defilement that its innate purity is conversely revealed. Just as a bright and clean mirror can manifest foul objects, and when foul objects appear, this conversely reveals the cleanliness of the mirror—how could these foul objects defile the mirror? If it did not manifest defilement, there would be no way to reveal its undefiled nature. “The essence of its cognition is unmoved”: because it was originally free of defilement and has now no new beginning of purity, the cognition of innate enlightenment has never shifted or moved. Furthermore, although it manifests defiled phenomena, it is not defiled by them; hence it is called “unmoved.” Just as the image in a mirror changes according to the physical object, yet the essence of the mirror itself has never moved. “Fully endowed with the undefiled, etc.”: within this innate enlightenment, the nature-virtues as numerous as the sands of the Ganges lack nothing. Furthermore, it serves as the cause of internal perfuming for sentient beings, causing them to weary of saṃsāra and joyfully seek nirvāṇa. Hence the Śrīmālādevīsiṃhanādasūtra says: “Because the tathāgatagarbha exists, one can weary of the suffering of saṃsāra and joyfully seek nirvāṇa.” The Foxing lun says: “The mind intrinsically pure by nature is called the truth of the path.” Furthermore, it is the karmic nature among the ten kinds of Buddha-nature.