Sri Aurobindo begins this new chapter exploring Mind, Dream and Hallucination with a citation from the Brihadaranyaka Upanishad: “This Self is a self of Knowledge, an inner light in the heart; he is the conscious being common to all the states of being and moves in both worlds. He becomes a dream-self and passes beyond this world and its forms of death….There are two planes of this conscious being, this and the other worlds; a third state is their place of joining, the state of dream, and when he stands in this place of their joining, he sees both planes of his existence, this world and the other world. When he sleeps, he takes the substance of this world in which all is and himself undoes and himself builds by his own illumination, his own light; when this conscious being sleeps, he becomes luminous with his self-light….There are no roads nor chariots, nor joys nor pleasures, nor tanks nor ponds nor rivers, but he creates them by his own light, for he is the maker. By sleep he casts off his body and unsleeping sees those that sleep; he preserves by his life-breath this lower nest and goes forth, immortal, from his nest; immortal, he goes where he wills, the golden Purusha, the solitary Swan. They say, “the country of waking only is his, for the things which he sees when awake, these only he sees when asleep”; but there he is his own self-light.”
There is a lot to consider in this Upanishad as relates to the subject of the mind and its states of consciousness, including the waking state, the dream state and the phenomenon of hallucination where the experience is other than what the world of physical reality presents, based on internal interpretation of perceived data. It is however essential to the process of acquiring Knowledge that we understand the tools of understanding, their functions, the various states of awareness, their limitations, their weaknesses and their deformations.
reference: Sri Aurobindo, The Life Divine, Book 2, Part I, Chapter 5, The Cosmic Illusion; Mind, Dream and Hallucination, pg. 412