The External Mind and Internal Mind in the Mahāsatipaṭṭhāna Sutta

When the Buddha expounded the five hindrances in the Great Mindfulness Sūtra, he spoke of inner greed and outer greed, inner ill will and outer ill will, inner sloth-and-torpor and outer sloth-and-torpor, inner restlessness-and-remorse and outer restlessness-and-remorse, and inner doubt and outer doubt. What do "inner" and "outer" each refer to?

If the mind is divided into inner and outer, the outer mind refers to the mind consciousness that sentient beings can generally discover and observe, while the inner mind refers to the mind root (manas), which sentient beings find difficult to discover and observe. The two conscious minds are one manifest and one hidden, one bright and one dark, one deep and one shallow; one is easily changed, while the other is firm and hard to change. Cultivation means first putting the practice into effect at the shallow level of the mind consciousness, and then at the deep level of the mind root—first preliminarily transforming the mind consciousness, and ultimately and completely transforming the mind root. Therefore, all afflictions and ignorance are, at first and most evidently, at the level of the mind consciousness, and, in the end and most hidden and profound, at the level of the mind root.

From this sūtra we can see that when the Buddha expounded the Small Vehicle dharma, although he did not explicitly speak of the dharma of the mind root, he nevertheless implicitly touched upon it—implicitly teaching that all afflictions and ignorance belong to the mind root, that severing afflictions means severing the afflictions of the mind root, that liberation is the liberation of the mind root, and that all dharmas must be put into effect at the mind root. If the problem of the mind root is resolved, all dharmas can be resolved, and one can ultimately and completely attain nirvāṇa and liberation.