There are thousands of texts referred to in Indian tradition as Upanishads. They all take this name to gain the authority of the original Upanishads, about 14 in number, which are considered part of the BRAHMANA or commentaries that are asso-ciated with the four VEDAS. (Each of the four is divided into a MANTRA portion, which consists of the chants themselves, and a BRAHMANA, or com-mentary. Both portions are considered SHRUTI, or revelation.)
Within the Brahmana portion of the Vedas are two classes of passages that later took the names ARANYAKA and Upanishad. The Aranyaka portions often closely resemble the Brahmana but often contain esoteric interpretations of the Vedic ritu-als. The further sections, sometimes within these Aranyaka sections, were termed Upanishads. While included in the Vedic literature, the Upanishads were composed somewhat later, roughly in the sev-enth to third centuries B.C.E., although they exist in the same ancient literary SANSKRIT.
Upanishad in its literal definition means “to sit down near.” They represented secret teachings reserved for those who sat near their GURU in the forest. They contain a wide range of material. Some of it is indistinguishable from the rest of the Brah-mana or Aranyaka, but other sections discuss the creation and nature of the universe. Most often, the discussions concern the BRAHMAN, or ultimate reality; the ATMAN, the ultimate self or soul; or their relationships with the individual self or soul.
In many places these Upanishads make clear that the individual self, seen from the highest consciousness, is nothing but the ultimate reality in all its glory. The exact relationship between the ultimate reality and the souls became the subject of centuries of discussion and mystic insights into the nature of things. The most commonly listed Upanishads are ISHA, KENA, KAT H A, Prashna, Mun-daka, MANDUKYA, TAITTARIYA, AITAREYA, CHANDO-GYA, BRIHADARANYAKA, Pingala, and Jabala.
Further reading: S. M. Srinivas Chari, The Philosophy of the Upanisads: A Study Based on the Evaluation of the Comments of Samkara, Ramanuja, and Madhva (New Delhi: Munshiram Manoharlal, 2002); S. N. Dasgupta, The History of Indian Philosophy. Vol. 1 (Delhi: Motilal Banarsidas, 1975); S. Radhakrishnan, trans., The Phi-losophy of the Upanisads (London: G. Allen & Unwin, 1935); ———, The Principal Upanishads (Atlantic Highlands, N.J.: Humanities Press, 1994).
Encyclopedia of Hinduism. A. Jones and James D. Ryan. 2007.