The Yogācārabhūmi-śāstra: Volume Thirty-Four (Part Twelve)

How should one contemplate the impermanent nature of decay arising from the stages of internal phenomena? By observing either oneself or others, from the youthful stage up to the aged stage, the continuous transformation of all conditioned phenomena manifests successive differences without resemblance. Having witnessed these phenomena, one should form the following thought: Truly, the nature of these conditioned phenomena is impermanent. For what reason? Because within the stages of the material body, the sequential changes and distinctions from youth to old age can indeed be directly perceived.

Explanation: How does one observe the impermanent nature of decay that appears in the stages of the material body? By observing oneself or others throughout the period from youth to old age, the continuous transformation of conditioned phenomena reveals successive distinctions without similarity. After observing these phenomena, one should hold this view: The nature of these conditioned phenomena is truly impermanent. Why is this conclusion reached? Because within the internal stages of the material body, the changes and differences from youth to old age are indeed observable in the present moment.