Is the consciousness with the power of introspection the mental consciousness accompanying the five sense consciousnesses or the solitary consciousness?

The power of introspection is precisely the self-validating cognition (sva-saṃvitti-saṃvitti-bhāga), which is one's own validation of oneself and one's own mental factors (caittas). At this time, one's own consciousness and its mental factors become the object-aspect. This object-aspect has nothing to do with the five sense objects; it is an Image-Only Object, and thus has nothing to do with the mental consciousness accompanying the five sense consciousnesses (pañcavijñāna-sahagata-manovijñāna); it is the object-aspect corresponding to the solitary mental consciousness. Therefore, the introspective power that possesses self-validating cognition belongs to the solitary mental consciousness, not to the mental consciousness accompanying the five sense consciousnesses. The key point for distinguishing between the two kinds of mental consciousness lies in whether the mental sense-object they correspond to is related to the five sense objects. If there is no relation, it is an Image-Only Object mental sense-object, which can only be discerned by the solitary mental consciousness.

When the mental consciousness accompanying the five sense consciousnesses appears, the solitary mental consciousness may also appear; the two kinds of mental consciousness can appear simultaneously. For example, when seeing a flower, the mental pondering, recollection, imagination, and so on related to the flower all belong to the solitary mental consciousness. The mental consciousness accompanying the five sense consciousnesses is only responsible for jointly discerning the flower before one's eyes together with the five sense consciousnesses; flowers of the past and the future can only be discerned by the solitary mental consciousness. This past and future includes one minute before and after—in short, anything that is not the present-moment object. For example, when seeing the person opposite you, the thoughts, impressions, and mental notions about this person all belong to the solitary mental consciousness. The mental consciousness accompanying the five sense consciousnesses has no thoughts or concepts; it cannot recall or anticipate, and the mental sense-objects it discerns cannot depart from the realm of the five sense objects.