Yogācārabhūmi-śāstra: Volume 34 (62)

Thus, the diligent practitioner of yoga observes the mental continuum transforming and differentiating, arising anew. Sometimes increasing, sometimes decreasing, temporarily existing, suddenly appearing, changing from moment to moment. This is impermanence. Observing the mental continuum entering into the aggregates of clinging, this is suffering. Observing the mental continuum apart from a second thing, this is emptiness. Observing the mental continuum arising from various conditions, not self-controlled, this is non-self. Thus it is called realizing the truth of suffering.

Thus, the diligent practitioner of yoga, observing the mental continuum continuously flowing and transforming, where a new phenomenon arises in the mind one moment, and another new phenomenon arises the next moment, constantly changing. Their mental states sometimes increase, sometimes decrease, all temporarily existing, appearing momentarily, changing afterward. Such a mind that changes from moment to moment is impermanent. Then, observing the mental continuum continuously, with clinging, it is grasped by the aggregates of clinging; this is suffering. Then, observing the mental continuum continuously, the mind does not belong to any phenomenon because the mind is empty; this is emptiness. Then, observing the mental continuum continuously, it arises from the combination of various conditions, not self-controlled, therefore it is non-self. Thus it is called realizing the truth of suffering. Impermanence, suffering, emptiness, and non-self are called the truth of suffering.