Yogācārabhūmi-śāstra: Volume 34 (24)

How to observe that all internal phenomena cease to appear entirely, revealing their nature of decay and impermanence? It means that by observing how, at a later time, even this skeletal state [of the body] no longer appears—having completely deteriorated, disintegrated, and vanished—every form of matter becomes invisible to the eye. Having witnessed this, one reflects thus: "Such conditioned formations are indeed impermanent in nature. Why? Because these forms undergo repeated alteration, and the phenomenon of transformation from prior to subsequent states is directly observable."

Explanation: How does one observe that all phenomena of the internal body cease to appear, completely extinguished, revealing the nature of decay and impermanence? One observes that even the skeletal remains of the body in a later period vanish entirely—utterly deteriorated, scattered, and ground to dust—such that every type of material form becomes invisible to the eye. After observing these phenomena, one gives rise to this thought: "Such conditioned formations are truly impermanent in nature. Why is this so? Because the forms of the corpse after death continuously undergo transformation, and the phenomenon of change from prior to subsequent states can be directly observed."