AUM—this whole world is that syllable! Here is a further explanation of it. The past, the present, and the future—all that is simply AUM, and whatever else that is beyond the three times, that also is simply AUM—for this brahman is the whole. Brahman is this self (atman); that (brahman) is this self (atman) consisting of four quarters.

The first quarter is Vaisvanara—the Universal One—situated in the waking state, perceiving what is outside, possessing seven limbs and nineteen mouths, and enjoying gross things.

The second quarter is Taijasa—the Brilliant One—situated in the state of dream, perceiving what is inside, possessing seven limbs and nineteen mouths, and enjoying refined things.

The third quarter is Prajna—the Intelligent One—situated in the state of deep sleep, which is when a sleeping man entertains no desires or sees no dreams; become one, and thus being a single mass of perception; consisting of bliss, and thus enjoying bliss; and having thoughts as his mouth. He is the lord of all; he is the knower of all; he is the inner controller; he is the womb of all—for he is the origin and the dissolution of beings.

They consider the fourth quarter as perceiving neither what is inside nor what is outside, nor even both together; not as a mass of perception, neither as perceiving nor as not perceiving; as unseen; as beyond the reach of ordinary transaction; as ungraspable; as indescribable; as one whose essence is the perception of itself alone; as the cessation of the visible world; as tranquil; as auspicious; as without a second. That is the self (atman), and it is that which should be perceived.

With respect to syllables, AUM is this very self (atman), whereas with respect to the constituent phonemes of a syllable, it is as follows. The constituent syllables are the quarters, and the quarters are the constituent phonemes, namely “a,” “u,” and “m.”

The first constituent phoneme—“a”—is Vaisvanara situated in the waking state, so designated either because of obtaining (apti) or because of being first (adimattva). Anyone who knows this is sure to obtain all his desires and to become the first.

The second constituent phoneme—“u”—is Taijasa situated in the state of dream, so designated either because of heightening (utkarsha) or because of being intermediate (ubhayatva). Anyone who knows this is sure to heighten the continuity of knowledge and to become common; and a man without the knowledge of brahman will not be born in his lineage.

The third constituent phoneme—“m”—is Prajna situated in the state of deep sleep, so designated either because of construction (miti) or because of destruction (apiti). Anyone who knows this is sure to construct this whole world and to become also its destruction.

The fourth, on the other hand, is without constituent phonemes; beyond the reach of ordinary transaction; the cessation of the visible world; auspicious; and unique. Accordingly, the very self (atman) is AUM. Anyone who knows this enters the self (atman) by himself (atman).

From the Mandukya Upanishad, Upanishads, translated by Patrick Olivelle, Oxford: Oxford University Press, 1996, pp. 289-290.