Book excerpt: The Upanishads are called shruti, revelations. Mandukya Upanishad, though the shortest of all Upanishads, is a revered, profound Upnaishad that teaches us about four states of consciousness: the waking, dreaming and sleeping states and Turiya, the fourth state, beyond the first three. In this Upanishad, there is a unique combination of psychology and profound philosophy, a clear exposition of the four states of consciousness.