Chandogya Upanishad is a part of Chandogya Brahmins, consisting of ten chapters. The first two chapters of this Brahmin are devoted to sacrifice ritual. The remaining eight are actually Chandogya Upanishad. According to some scholars, Chandogya Upanishad is the oldest of the Upanishads, dating back to the period of the drafting of Brahman (the beginning I millennium BC). The special importance of this Upanishad in Vedantic philosophy is evident due to the references to it in the Vedanta-sutra (Fieser and Powers 18).
While in the Brahmanas the primary was recognized an appeal to the forces of nature (the “gods”ť) through the way of action (karma marga), i.e. primal was the act of sacrifice itself, in the Chandogya Upanishad attention of the authors was drawn not to external rituals, but to “internal”ť sacrifices in thoughts and in souls. The importance of word, speech, reflection was growing. According to the authors of Chandogya Upanishads, everything in the world is mortal, even the gods. And only being expressed in words, finding an asylum in them, the mortal becomes immortal (Chandogya Upanishad).
In Chandogya Upanishad, the syllable OM, which consists of three letters (sounds) a, u, m, symbolized the three Vedas – Rigveda, Sama veda, and Yajurveda. The OM syllable is called udgitha ”“ an anthem of Sama veda performed out loud. In the following sections udgitha receives a broader interpretation. Udgitha is regarded as something noncontradictory, uniform and eternal (Fieser and Powers 22). For example, neither breathing through the nose, nor speech can be considered as udgitha, as nose perceives both pleasant and unpleasant scents, and people say both the truth and lies. But mouth breathing should be reputed as udgitha because it does not perceive anything contradictory. It is indeed udgitha, or atman (Chandogya Upanishad).
The passages of Chandogya Upanishad are devoted mainly to the interpretation of one of the basic concepts of ancient Indian philosophy – the concept of Brahman (substance of ultimate reality). The universal property of “support”ť is here attributed to Brahman, as everything generates from Brahman, and then turns backs into it.
Chandogya Upanishad associates the concept of Brahman with the concept of atman. Atman is regarded as the essence of the eternal and immutable substance (ageless, undying Atman), but still an active though it cannot be touched (as salt dissolved in water) (Chandogya Upanishad): in accordance with its true desires and designs, Atman is constantly active in this world, especially in the human body (the principle of Atman’s oneness).
The concept of Atman, the world-soul, developed from the concept of peace-Purusha (in the Rigveda) and the early concept of a personal creator (Prajapati), eventually grows in the Upanishads into to the impersonal reason of all things – Brahman. Brahman appears as a materialized force in all existing things that creates, maintains, retains, and returns all the worlds and all the nature back to itself (Fieser and Powers 24-25).