The Aitareya Upanishad is part of the Aitareya Aranyaka in the Rig Veda. It comprises the fourth, fifth and sixth chapters of the second book of Aitareya Aranyaka. It explains the inner or symbolic meaning of the sacrifice rituals described in the previous chapters of the Aranyaka. Particularly famous is the maha vakya ("great aphorism") prajnanam brahma (3.3), "Brahman is perfect knowledge", considered the essence of the Rig Veda. Aitareya Upanishad discusses three philosophical themes:- first, that the world and man is the creation of the Atman (Soul, Universal Self); second, the theory that the Atman undergoes threefold birth; third, that Consciousness is the essence of Atman. The Aitareya Upanishad is a short prose text, divided into three chapters, containing 33 verses (First Adhaya - 1st Khand 4 verses, 2nd Khand 5 verses, 3rd Khand 14 verses; Second Adhaya - 6 verses; Third Adhaya - 4 verses).
Atman existed alone prior to the creation
In the first chapter of the Aitareya Upanishad, Atman is asserted to have existed alone prior to the creation of the universe. It is this Atman, the Soul or the Inner Self, that is then portrayed as the creator of everything from itself and nothing, through heat. The text states that the Atman created the universe in stages. First came four entities: space, maram (earth, stars), maricih (light-atom) and apas (ur-water, cosmic fluid). After these came into existence, came the cosmic self and eight psyches and principles (speech, in-breathing, sight, hearing, skin/hair, mind, out-breathing, reproductivity). Atman then created eight guardians corresponding to these psyches and principles. Then came the connective principles of hunger and thirst, where everything became interdependent on everything else through the principle of apana (digestion). Thereafter came man, who could not exist without a sense of Self and Soul (Atman). But this sense then began cogitating on itself, saying that “I am more than my sensory organs, I am more than my mind, I am more than my reproductive ability”.
Atman is born thrice
In the second chapter, Aitareya Upanishad asserts that the Atman in any man is born thrice: first, when a child is born (procreation); second, when the child has been cared for and loved to Selfhood where the child equals the parent; third, when the parent dies and the Atman transmigrates. The overall idea of chapter 2 of Aitareya Upanishad is that it is procreation and nurturing of children that makes a man immortal, and the theory of rebirth, which are the means by which Atman sustainably persists in this universe.
Nature of Atman
Aitareya Upanishad, like other Upanishads of Hinduism, asserts the existence of Consciousness as Atman, the Self or Brahman. It contains one of the most famous expressions of the Vedanta, “Prajnanam Brahma” (Knowledge is Brahman/god/divine/holy), which is one of the Mahāvākyas