What kind of "knowing" is "right knowing"?

Question: There is a "knowing" in our mind that is an unchanging observer—is it the manas? Is it the same as the "knowing" in dreams? Why does it disappear when we fall asleep? Is maintaining this luminously aware "knowing" right mindfulness (正知)?

Answer: This knowing in the mind is subject to change and impermanence; in many situations it ceases to know. Therefore it is the knowing nature of the impermanent, arising-and-ceasing mind consciousness. This knowing also exists in dreams, and it too is the knowing of the mind consciousness. However, on all dharmas there is also simultaneously the knowing of the manas; it is just that the manas is hidden extremely deeply, and its knowing nature is very hard to discover.

Maintaining this knowing nature and being clear about the nature of the six sense objects belongs to methods of cultivating meditative concentration (samādhi); it is not seeing the Path. If it is not the knowing of the mind consciousness at the level of seeing the mind, then the knowing that falls within the five aggregates is not right mindfulness, because it does not know the eighth consciousness and does not know the operation of the eighth consciousness; it only knows conditioned, arising-and-ceasing dharmas and does not know the unarisen and unceasing dharma—this is the mind consciousness's lack of right mindfulness. True right mindfulness is the knowing nature of the eighth consciousness, the knowing nature of the tathāgatagarbha, which does not fall within the five aggregates, the twelve sense bases, or the eighteen realms. He has no outflows, no ignorance, and no impermanent mutability of arising and ceasing. This is true rightness—thorough and ultimate rightness—which need not be born and will never cease.