Mandukya Upanishad
The Mandukya Upanishad describes three states of consciousness, namely waking (jågrat), dreaming (svapna), and deep sleep (suṣupti), and 'the fourth', beyond and underlying these three states:
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The first state is the waking state, in which we are aware of our daily world. "It is described as outward-knowing (bahish-prajnya), gross (sthula) and universal (vaishvanara)". This is the gross body.
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The second state is the dreaming mind. "It is described as inward-knowing (antah-prajnya), subtle (pravivikta) and burning (taijasa)". This is the subtle body.
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The third state is the state of deep sleep. In this state the underlying ground of consciousness is undistracted, "the Lord of all (sarv'-eshvara), the knower of all (sarva-jnya), the inner controller (antar-yami), the source of all (yonih sarvasya), the origin and dissolution of created things (prabhav'-apyayau hi bhutanam)". This is the causal body.
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Turiya, "the fourth," is the background that underlies and transcends the three common states of consciousness. In this consciousness both absolute and relative, saguna brahman and Nirguna Brahman, are transcended. It is the true state of experience of the infinite (ananta) and non-different (advaita/abheda), free from the dualistic experience which results from the attempts to conceptualise ( vikalpa) reality. It is the state in which ajativada, non-origination, is apprehended.
The Mandukya Upanishad opens by declaring, "Aum!, this syllable is this whole world". Thereafter it presents various explanations and theories on what it means and signifies. This discussion is built on a structure of "four fourths" or "fourfold", derived from A + U + M + "silence" (or without an element.
Aum as all states of time
In verse 1, the Upanishad states that time is threefold: the past, the present and the future, that these three are "Aum". The four fourth of time is that which transcends time, that too is "Aum" expressed.
Aum as all states of Atman
In verse 2, states the Upanishad, "this brahman is the Whole. Brahman is this self (ātman); that [brahman] is this self (ātman), consisting of four corners."
Aum as all states of consciousness
In verses 3 to 6, the Mandukya Upanishad enumerates four states of consciousness: wakeful, dream, deep sleep and the state of ekatma (being one with Self, the oneness of Self). These four are A + U + M + "without an element" respectively.
Aum as all of etymological knowledge
In verses 9 to 12, the Mandukya Upanishad enumerates fourfold etymological roots of the syllable "Aum". It states that the first element of "Aum" is A, which is from Apti (obtaining, reaching) or from Adimatva (being first). The second element is U, which is from Utkarsa (exaltation) or from Ubhayatva (intermediateness). The third element is M, from Miti (erecting, constructing) or from Mi Minati, or apīti (annihilation). The fourth is without an element, without development, beyond the expanse of universe. In this way, states the Upanishad, the syllable Aum is the Atman (the self) indeed.