Yogācārabhūmi-śāstra: Volume 34 (20)

How to contemplate the impermanent nature of internal phenomena as they undergo transformation through contact? It means that when, through contact with objects conducive to pleasant feelings, one experiences the pleasant feeling arising from the condition of pleasant contact, one naturally discerns the state of the pleasant feeling. Just as one discerns the state of the pleasant feeling, so too should one understand the discernment of the state of painful feeling and the state of neither-painful-nor-pleasant feeling. By discerning the successive transformations of these feelings—their ever-renewing nature, not abiding as before; their increase or decrease; their temporary existence, suddenly arising and then swiftly changing and ceasing—upon knowing this, one forms this thought: "All such conditioned phenomena possess the nature of impermanence."