Chandogya Upanishad

"The Sama-Veda includes among its treasures the Chhandogya Brahmana, consisting of ten parts; of these, the last eight constitute the Chhandogya Upanishad. In turn, the eight parts of the Upanishad may be broadly divided into two sections.

The first, consisting of five parts, deals with upasana, or ritualistic worship with emphasis on meditation. The second section, of three parts, discusses certain fundamental doctrines of the Vedanta philosophy: in the sixth part, the Vedantic dictum “Tat Tvam Asi,” or “That Thou Art”; in the seventh part, the doctrine of Bhuma, or Infinity; and in the eighth part, the doctrine of Atman.

The Brihadaranyaka and the Chhandogya, which are the longest of the Upanishads, occupy a superior position among the Upanishads known to us. Discussing profound philosophical truths through numerous anecdotes, they form the basis of the later development of the Vedanta philosophy. Sankaracharya, in establishing the philosophy of nonduality, derived support from such statements of the Chhandogya Upanishad as: “One only without a second” (VI. xiv. 1), “From It the universe comes forth, into It the universe merges, and in It the universe breathes. Therefore a man should meditate on Brahman with a calm mind” (III. xiv. 1.), and “That is the Self. That thou art” (VI. viii. 7). If a serious student carefully reads the Chhandogya Upanishad with the help of Sankaracharya’s commentary, he will come to know all the major topics of the Upanishads and will be directed toward the philosophy of the inscrutable Brahman."

- Swami Nikhilananda