Why Can One Know One Is Dreaming Within a Dream?

When dreaming, there are three: the eighth consciousness, the manas, and the solitary consciousness (mano-vijñāna-eka-citta). The one that governs the dream is the manas. The manas may be lucid or muddled, and the solitary consciousness may be lucid or muddled. The factors that determine whether the sixth and seventh consciousnesses are lucid or muddled lie in their meditative concentration and cognitive wisdom. Usually, practitioners whose meditative concentration is not very good are often not very lucid in dreams; while dreaming, they do not know they are dreaming. Those who can know the dream within the dream are generally clear-headed and possess some degree of meditative concentration. Who is the one that knows the dream within the dream? The observer observes the dream, and the master of the dream is the manas; this observer is the solitary consciousness. It is relatively lucid within the dream and does not follow the dream's turning, and therefore it knows that it is in a dream and knows the dream.

When the solitary consciousness is not clear, it follows the dream's turning within the dream, muddled and confused, taking the dream to be real. When the solitary consciousness is clear, the manas is generally also clear, and one can be master of oneself within the dream, knowing wholesome and unwholesome, true and false, good and bad (kuśala-akuśala, satya-mithyā, śubha-aśubha), refraining from creating unwholesome karma, and even knowing to engage in recollection of the Buddha (buddhānusmṛti) and so on—not much different from daytime. If one can be master of oneself within the dream, then at the time of death one can have some assurance and not drift along with karma. If one cannot be master of oneself within the dream and turns along with karma, then in the neutrino-body (antarā-bhava) one will be even less able to be master of oneself, and the karmic obstructions will govern, causing one to drift along. Within the dream, the solitary consciousness and the manas also mutually influence each other: the manas can still govern the solitary consciousness, and the solitary consciousness can still influence the manas. Therefore, when one knows the dream within the dream, both the manas and the mind consciousness are relatively lucid.