The Mandukya Upanishad marks a pivotal moment in the history and evolution of human thought. In it, we are treated to the first description of the fundamental four “states” of consciousness. Waking, dreaming, dreamless sleep, and Turiya, simply “the fourth.” The final state represents the liberated state. A kind of consciousness wherein consciousness itself becomes its own object. This is the pure, non-dual awareness that greets the patient mystic.
For more on the Mandukya Upanishad: I first encountered it in the pages of Evan Thompson’s book – Waking, Dreaming, Being: http://amzn.to/2p5UoIx
Or, I also referenced this book for this video: Om the Eternal Witness: Secrets of the Mandukya Upanishad by Swami Rama: http://amzn.to/2p5UIGX