金光明經文句記 3.45

3.45

三、「陰之」下，明三身體用相。法報雙非，全體起用，能常無常。常用則長等者，長則四喻、短則八十，既皆應壽，悉是無常。今以長短分常無常者，由山斤等能顯於常。若非體常，安令應用人天莫數？是故信相聞八十滅，疑壽無常；聞四喻長，悟常住體。故云常用則長，無常用則短。

Third, from 'Of the skandhas' downward, clarifying the characteristics of the substance and function of the three bodies. Dharma and retribution are both doubly negated; their entire substance generates function, capable of permanence and impermanence. When the permanent is functioning, it is long, etc.: long is the four metaphors, and short is eighty [years]. Since both are response lifespans, all are impermanent. Now dividing permanent and impermanent by long and short is because the weight of the mountain and so forth can reveal permanence. If the substance were not permanent, how could the responsive function be innumerable to humans and devas? Therefore, when Ruciraketu heard of cessation at eighty, he doubted that the lifespan was impermanent; when he heard the length of the four metaphors, he awakened to the permanently abiding substance. Thus it is said that when the permanent functions, it is long, and when the impermanent functions, it is short.