Aitareya Upanishad (thing)

This is one of the oldest of the Upanishads, part of the Rig Veda, and is correspondingly one of the most difficult to read, due to its highly ritual and symbolic language. Like the Brihdaranyaka Upanishad, its opening chapters deal with the process of the creation of the universe, but whereas the Brihadaranyaka equates the universe to the sacrificial horse of the ashvamedha, the Aitareya shows the emanation of the different parts and elements of the universe from the Self. It also deals with the creation of food, which is to be understood not just as physical nourishment but a representation of the sustaining and feeding force inherent in nature.

The later sections of the Aitareya deal with the Self, its characteristics (or lack thereof), and the philosophical method by which seekers after truth can attain knowledge of their own nature. One of the Mahavakyas (great statements) of the Upanishads is found here: Brahman is Intelligence (also translated as Brahman is Knowledge). As one of the founding texts of Vedanta, the Aitareya Upanishad, along with the Brihadaranyaka, assert the power of native human understanding and intelligence as a means to enlightenment. This leads to the path of Yoga known as jnana (knowledge), characterised by the mantra or axiom neti, neti. This means 'Not this, not that', and in the closing sections of the Aitareya Upanishad the Self is identified as the true essence of all identifiable things, without being to any of them:

Who is he whom we meditate on as the Self? Which is the Self?
That by which we see (form), that by which we hear (sound), that by which we perceive smells, that by which we utter speech, that by which we distinguish sweet and not sweet, and what comes from the heart and the mind, namely, perception, command, understanding, knowledge, wisdom, seeing, holding, thinking, considering, readiness (or suffering), remembering, conceiving, willing, breathing, loving, desiring?
No, all these are various names only of knowledge (the true Self).
And that Self, consisting of (knowledge), is Brahman

The Aitareya is tough going, but if the reader can wade through the more stylized and repetitive parts, it becomes clear that the writers are speaking from the same perspective as the writers of most of the Upanishads, and are teaching the same Vedantic lesson of adherence to and meditation on the Highest Self. It's something that modern people are now familiar with, as we have heard these things said by thousands of 'teachers', in thousands of books and journals and TV programs in the 20th Century, but to see it written in a text thousands of years old is quite amazing.