Mandukya Upanishad in The Thirteen Principle Upanishads -- London: Oxford University Press, 1954

The Mandukya Upanishad is the shortest of all the principle Upanishads. It is a prose work composed of less than 500 words. It begins by defining the word Om as "this whole world...the past, the present, the future...and whatever else that transcends threefold time." It goes on to explain that this self (atman) is Brahma and is exists in four states: waking, dreaming, deep sleep, and a mysterious fourth state that is without any distinctive description, but is the state of being one with the Self (the atman.) The word Om (Aum) is then described as representing these four states: A is the waking state, U is the dream state, M is the state of deep sleep, and the silence which follows the utterance of the word is the final state of being one with the Self.