The Aitareya Upanishad belongs to the Rigveda and forms the fourth, fifth and sixth chapters of the second book of the Aitareya Aranyaka, which tradition credits to Rishi Mahidasa Aitareya. It is a short prose text of thirty-three verses in three chapters.
It begins before the world: in the beginning the Self alone was, and it resolved to create the worlds and then their guardians. The deities it brings forth ask for a home; offered a cow and then a horse, they refuse them, but shown a human being they call it well made and enter it — fire becomes speech and enters the mouth, wind becomes breath in the nostrils, the sun becomes sight in the eyes, and the moon becomes mind in the heart. The Self enters the body by the crown of the head, through the door called vidriti. The second chapter tells of the three births of the self, and of Rishi Vamadeva, who knew the births of the gods while still in the womb. The third asks which is the Self — that by which one sees, hears, smells and speaks — and answers that all is guided by consciousness: prajnanam brahma, consciousness is Brahman.
Prajnanam brahma is counted among the four great sayings, the mahavakyas, one drawn from each Veda. It is the great saying of the Govardhana Matha at Puri, home of the Jagannath temple — the matha of Adi Shankaracharya's tradition that holds the Rigveda. The Aitareya opens with the peace chant vang me manasi — may my speech rest in my mind and my mind in my speech — with its vow to speak what is right and what is true. It comes eighth in the Muktika Upanishad's list of the ten principal Upanishads.