Yogācārabhūmi Śāstra, Volume 34 (12)

Original text:云何寻思内事分位所作。变坏无常之性。谓由观见或自或他。从少年位乃至老位。诸行相续。前后差别。互不相似。见是事已。便作是念。如是诸行。其性无常。何以故。此内分位。前后变异。现可得故。

Explanation: How does one contemplate the nature of decay and impermanence arising in the stages of the material body? By observing oneself or others, from the stage of youth through to the stage of old age, the continuous arising and changing of the volitional formations, and the differences between before and after, with nothing similar between them. After observing these phenomena, one should hold this notion in mind: the nature of these volitional formations is truly impermanent. Why is this conclusion reached? Because in the internal stages of the material body, the changes and differences from youth to old age can indeed be directly observed in the present.