大乘起信論義記 3.489

3.489

初、無常觀。二、一切心行下，明苦觀。三、應觀下，明無我觀。於中過去無體難追，現在剎那不住，當來本無積聚，但緣集歘有，不從十方來故也。

First is the contemplation of impermanence. Second, from 'all mental activities' onward, clarifies the contemplation of suffering. Third, from 'One should contemplate' onward, clarifies the contemplation of non-self. Therein, the past has no substantiality and is hard to pursue; the present does not abide for a kṣaṇa; the future originally has no accumulation, but only suddenly exists through the assembly of conditions, not having come from the ten directions.