Why is it possible to be aware in a dream that one is dreaming?

During dreaming, there are three elements: the eighth consciousness, the manas, and the isolated sixth consciousness. The master of the dream is the manas, which may be either lucid or muddled. The isolated sixth consciousness may also be lucid or muddled. The factors determining whether the sixth and seventh consciousnesses are lucid or muddled lie in the meditative power and wisdom cognition of these two. Generally, practitioners with insufficient meditative power tend to be less lucid in their dreams, unaware that they are dreaming. Those who can recognize that they are dreaming while dreaming are usually clear-minded and possess some level of meditative power. Who is it that knows the dream in the dream? The observer witnessing the dream is the isolated sixth consciousness, while the master of the dream is the manas. This observer—the isolated sixth consciousness—remains relatively lucid within the dream, not swept away by the dream, thus enabling it to know that it is dreaming and to recognize the dream itself.

When the isolated sixth consciousness is not lucid, it muddles along with the dream, taking the dream as reality. When the isolated sixth consciousness is lucid, the manas is generally lucid as well, enabling one to maintain control during the dream, knowing good and evil, right and wrong, refraining from creating unwholesome karma, and even engaging in practices like chanting the Buddha’s name, with little difference from the waking state. Only if one can maintain control in dreams can one have some assurance at the moment of death, avoiding being swept away by karma. If one cannot maintain control in dreams and is carried along by karma, then during the bardo state, one will be even less able to maintain control, and karmic obstacles will take charge, leading to drifting. Within the dream, the isolated sixth consciousness and the manas also influence each other: the manas still governs the isolated sixth consciousness, while the isolated sixth consciousness still affects the manas. Therefore, when one knows the dream while dreaming, both the manas and the sixth consciousness are relatively lucid.