The mental factors (caitta) are the modes of activity of the mind king (citta).

The five universally operative mental factors (caittas) do not merely operate when thoughts arise and stir; rather, in every instant and every moment they operate together with the eight consciousnesses, accompanying the eight consciousnesses without ever departing for even an instant. Whenever the eight consciousnesses appear, the five universally operative mental factors accompany and operate alongside them. As stated in the first volume of the Yogācārabhūmi-śāstra, the five universally operative mental factors are the aids and companions of the mind-kings (citta-rāja). Whenever a mind-king operates, it takes the five universally operative mental factors as its aids and companions; without the five universally operative mental factors, the mind-king cannot operate.

Regarding the operation of the mental factor of mental engagement (manaskāra): for example, before a consciousness seed is about to be born, there must be a direction in which it is born, and the place where it will be born must be determined—for instance, being born upon the flower-color before one's eyes. The manas needs to discern the flower-color, and once the location of this dharma has been determined, the consciousness seed rushes forth and is born at the place of that flower's material sense-object at the supreme faculty (rūpa-prasāda); or it rushes toward the location of a certain sound and is born at the place where the sound sense-object appears at the supreme faculty; or it is born at some other location among the six sense-objects that the manas wishes to discern. Therefore, before a consciousness seed is born there is mental engagement (manaskāra)—mental engagement means having a direction of birth, a location of birth. The mental factor of mental engagement begins to operate before the consciousness seed is born, and after the birth it must continue to operate without cease. The operation-marks of the mental factors and the mind-king merge together perfectly: the mental factors are precisely the operation-marks of the mind-king. On the surface, what manifests are the operation-marks of the mental factors, yet hidden within them are the operation-marks of the flowing forth of the mind-king's consciousness seeds. Although the two are in accord, each has its own operation-marks.

The last of the five universally operative mental factors is the mental factor of volition (cetanā). The final function of the mental factor of volition is action and fabrication. Once the mental factor of volition has completed its fabrication, it is equivalent to the mind-king having completed its fabrication; the consciousness-mind then ceases or shifts its location, and the mental factors at that dharma-location likewise disappear. From this principle, the operation-marks of the mind-king and the mental factors are in accord: their operations are twisted together as one. The operation-marks manifest in the operation-marks of the mental factors, within which are hidden the operation-marks of the flowing forth of the consciousness seeds—they cannot be separated from one another.

When the consciousness seeds flow forth, they form the substance of the mind-king. The substance of the mind-king is pure, because the consciousness seeds are pure. The consciousness seeds have no defiled nature, nor any wholesome or unwholesome nature; but when they combine with the mental factors and operate in the form of mental factors, mental actions that are wholesome, unwholesome, or neither wholesome nor unwholesome appear. Therefore, the mental actions of the eight consciousness mind-kings and the mental factors are in fact one and the same: the mental factors are the mental actions of the mind-kings. When the eight consciousness mind-kings operate, they operate in the form of mental factors—especially in the form of the five universally operative mental factors.