Yogācārabhūmi-śāstra: Volume 34 (25)

Thus, by the power of intensified mental attention based on direct perception, one first observes the various changing and impermanent natures within the fifteen internal phenomena. Having observed these, one then proceeds to observe the various changing and impermanent natures within the sixteen external phenomena. How does one observe the changing and impermanent nature of earth phenomena? It is done by observing this specific location: previously, no meditation sites, celestial temples, dwellings, marketplaces, city walls, or similar structures were established here. Later, one sees newly built structures that are well-constructed and well-adorned. Then, at another time, one sees those same structures become decayed, dilapidated, crumbling, scattered, collapsed, ruined, breached, burned by fire, or swept away by water. Having witnessed these events, one generates this thought: "Such conditioned phenomena truly possess an impermanent nature." Why is that? Because the transformations in their form and appearance, from before to after, are observable right now.

Explanation: Thus, through the power of intensified mental attention based on direct perception, one observes the various changing and impermanent natures within the fifteen internal phenomena, and subsequently continues to observe the various changing and impermanent natures within the sixteen external phenomena. How does one observe the changing and impermanent nature of earth phenomena? One observes the location of this land: previously, meditation sites, temples, dwellings, marketplaces, city walls, and so forth had not been built there. Later, these structures were newly built on this land. Then, at a later time, one sees these structures become old and decayed, broken down, scattered, dilapidated, collapsed, ruined, burned by fire, or washed away by water. Having observed these things, one generates this mental reflection: "Such conditioned phenomena truly possess an impermanent nature." Why is this said? Because the transformation in their form and appearance, from before to after, is observable right now.