Yogācārabhūmi Śāstra, Fascicle 34 (20)

Original text:云何寻思内事触对。所作变异。无常之性。谓由触对顺乐受触。领乐触缘。所生乐时。自能了别乐受分位。如能了别乐受分位。如是了别苦受分位。不苦不乐受分位。应知亦尔。彼由了别如是诸受。前后变异。是新新性。非故故性。或增或减。暂时而有。率尔现前。寻即变坏。知是事已。便作是念。如是诸行。其性无常。

Explanation: How does one contemplate the impermanent nature of the changes that occur when internal matters come into contact and confrontation? When one touches a pleasant feeling-contact that accords with the mind, and receives the pleasant feeling born from the condition of that pleasant contact, one can naturally discern the stage of pleasant feeling. Just as one can discern the stage of pleasant feeling, so too should one discern the stage of painful feeling, and likewise the stage of neither-painful-nor-pleasant feeling. Because one discerns the successive changes of these three kinds of feeling, the feelings are constantly renewed and transformed, never remaining fixed as they were at the beginning; the three kinds of feeling sometimes increase and sometimes decrease. Each kind of feeling exists only temporarily, suddenly appearing and soon arising and perishing. After observing these phenomena, the following thought arises in the mind: such conditioned formations are truly impermanent in nature.