Yogācārabhūmi-śāstra: Volume 34 (Part 13)

How should one contemplate the characteristic of change and impermanence manifested in the internal manifest form? By observing that either oneself or others previously possessed a splendid complexion, with skin fresh and lustrous, but later encountered an unpleasant complexion, with skin withered and dry. Then, at a subsequent time, one again sees a splendid complexion, with skin fresh and lustrous. Having witnessed these events, one thinks thus: "Such conditioned phenomena are indeed impermanent in nature. Why is that? Because this internal manifest form undergoes successive changes, and this transformation is observable right now."

Explanation: How does one observe the characteristic of change and impermanence arising in the manifest form perceived by the eye-consciousness within the internal body? One observes that oneself and others originally had a beautiful skin tone, with skin tender and lustrous, but later finds the skin tone deteriorated—withered and no longer fresh and lustrous. After some time passes, one again sees the complexion become beautiful and the skin lustrous. Having observed these phenomena, one thinks: "All conditioned phenomena like this are indeed impermanent in nature. Why is this said? Because these internal manifest forms differ before and after, undergoing change and impermanence, which can be observed directly right now."