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 perishable and impermanent nature arising from the stages and phases of the internal phenomena (of the body)? By observing oneself or others, from the stage of youth all the way to the stage of old age, the formations continue in succession and undergo transformation, with differences between earlier and later, none resembling the others. After observing these phenomena, one should form this understanding in the mind: the nature of these formations is truly impermanent. Why is this conclusion reached? Because within the internal stages and phases of the material body, the changes and differences from youth to old age are indeed directly observable in the present.