The Aitareya Upanishad is a short prose text, divided into three chapters, containing 33 verses. It comprises the fourth, fifth and sixth chapters of the second book of the older vedic text, Aitareya Aranyaka.
The rishi of the Aitareya Aranyaka and the Aitareya Brahmana is Aitareya Mahidasa. In Chandogya Upanishad, Aitareya Mahidasa is said to have lived for 116 years (Chhandogya 3.16.7).
In the first chapter of the text, Atman, the inner self, is portrayed as a divine creator. In the second chapter, the three births of the Atman are described. The third chapter deals with the qualities of the self or Brahman. It contains one of the most famous expressions of the Vedanta, "Prajnanam Brahma", which is one of the Mahāvākyas.
^See Henry Thomas Colebrooke (1858), Essays on the religion and philosophy of the Hindus. London: Williams and Norgate. In this volume, see chapter 1 (pp. 1–69), On the Vedas, or Sacred Writings of the Hindus, reprinted from Colebrooke's Asiatic Researches, Calcutta: 1805, Vol 8, pp. 369–476. A translation of the Aitareya Upanishad appears in pages 26–30 of this chapter.