There are four different states of consciousness.
The Mandukya Upanishad is the first to describe a hierarchy of levels of experience.
The upanishad progresses from maximum duality to absolute nonduality in its depiction; these four stages are also connected to the phonetic constituents of the holy sound Om.
Om represents the four levels of consciousness and is the sum and substance of all of them.
The initial stage of consciousness is awake consciousness, in which one experiences both subject and object; next comes dreaming, in which one's experience is completely subjective; and finally, profound sleep, in which one loses all awareness of oneself as subject (until waking).
Because the last condition is so far away from human experience that it cannot be described in words, it is simply referred to as "the fourth" (turiya).
This fourth condition is explicitly recognized as the ultimate truth, the atman or inner Self, knowledge of which provides complete freedom of the soul in the Mandukya Upanishad.
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