What Is the Inner and Outer Mind-Consciousness?

When the Buddha spoke about the five hindrances in the Mahāsatipaṭṭhāna Sutta, he mentioned internal sensual desire and external sensual desire, internal ill-will and external ill-will, internal sloth-and-torpor and external sloth-and-torpor, internal restlessness-and-worry and external restlessness-and-worry, internal doubt and external doubt. What do "internal" and "external" respectively refer to?

If the mind is divided into internal and external, the external mind refers to the consciousness that sentient beings can generally perceive and observe, while the internal mind refers to the mental faculty (mano) that sentient beings find difficult to perceive and observe. These two types of consciousness are distinct: one is apparent, the other hidden; one is manifest, the other subtle; one is superficial, the other profound; one is easily transformed, the other is deeply entrenched and difficult to change. Spiritual practice is first applied to the superficial level of consciousness, then to the profound level of the mental faculty. Initially, consciousness is transformed preliminarily, and ultimately, the mental faculty is transformed completely. Therefore, all afflictions and ignorance are initially and most evidently at the level of consciousness, but ultimately and most profoundly hidden at the level of the mental faculty.

From this sutra, we can see that when the Buddha taught the Hinayana Dharma, although he did not explicitly mention the Dharma pertaining to the mental faculty, he implicitly addressed it. He implied that all afflictions and ignorance belong to the mental faculty, that eliminating afflictions means eliminating the afflictions of the mental faculty, that liberation is the liberation of the mental faculty, and that all Dharma practices must be applied to the mental faculty. By resolving the problems of the mental faculty, all Dharma matters are resolved, ultimately leading to the complete attainment of Nirvana and liberation.