大乘起信論略述 2.221

2.221

論。一切心行至以是苦　述曰。此明修觀除樂倒也。言心行者。謂彼四蘊體遷流故。以有逼迫壞疾遷流總別苦性故。名為苦。

[Treatise:] All mental activities [... therefore it is suffering]. [Exposition:] This clarifies cultivating insight to remove the inversion of pleasure. "Mental activities" refers to the four aggregates, because their essence flows and changes. Because they have the general and specific natures of suffering—oppression, decay, swiftness, and flux—they are called suffering.