大乘起信論義記 2.187

2.187

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.