Commentary by Swami Chinmayananda -
Of the ten important Upanishads, this Upanishad is the briefest and most commented upon. No study of this Upanishad is complete without the study of the Karika by Sri Gaudapada. It exhaustively describes the four quarters of the atman – waking, dream, deep sleep and turiya. The last being exhaustively and completely defined in the language of negation.