大乘起信論義記 2.169

2.169

問：始得自利已方起利他業，云何利他說無始耶？答：有二釋：一云、如來一念遍應三世，所應無始故，能應則無始，猶如一念圓智遍達無邊三世之境，境無邊故，智亦無邊，無邊之智所現之相，故得無始亦能無終，此非心識思量所測，故名不思議業也。二云、以無明盡故，始覺即本，然彼本覺無始世來常起業用益眾生故，始覺同彼，故亦無始。以一切佛無差別故、無新舊故、皆無始覺之異故、本覺平等無始無終故，故能常化眾生，是真如之用故，云不思議業也。

Question: Only after first attaining self-benefit does one initiate the activity of benefiting others; why is benefiting others spoken of as beginningless? Answer: There are two explanations: First, it is said that in a single thought the Tathāgata responds pervasively to the three times. Because that which is responded to is beginningless, that which can respond is beginningless, just as the complete cognition of a single thought pervasively penetrates the limitless realms of the three times. Because the realms are limitless, the cognition is also limitless; and because the marks manifested by limitless cognition are such, they are able to be beginningless and also able to be endless. This cannot be measured by the deliberations of the mind and consciousness, and is therefore called inconceivable activity. Second, it is said that because avidyā is exhausted, incipient enlightenment is identical with the innate; yet because that innate enlightenment has constantly initiated activities and functions to benefit sentient beings from beginningless ages, incipient enlightenment is identical with it and is therefore also beginningless. Because all buddhas have no distinction, have no new or old, and all lack any difference of incipient enlightenment, and because innate enlightenment is equal, without beginning and without end, they can constantly transform sentient beings; this is the function of tathatā, and is therefore called inconceivable activity.