UPANISAD. — U. signifies “esoteric doctrine,” secret, to be communicated only to a few initiates. It is the common designation of a series of works of diverse age, size, and character, but all equally intended to Brahman (v.), the cosmic principle, and on its identity with Atman, the “self,” the psychic principle, the soul. Canonical are only the U. composed within the Vedic schools (śākhā), of which they are the final product. They are therefore collectively called also Vedānta, “end of the Veda,” a name that was likewise given to the monism of Bādarāyana and Śaṅkara, founded on the Upanisadic doctrines (v. VEDĀNTA).
Annexed to the Brāhmaṇa (v.) by means of the Āraṇyaka (v. BRAHMANESIMO), the U. constitute their epilogue, substituting philosophy for the allegorical interpretation of ritual. The formation of the new doctrine is usually assigned to a period of about three centuries (800–500 B.C.), but it is not possible to determine the age of the individual U. Following Prof. Deussen, those enjoying the reputation of canonical texts and considered revelation (śruti) are divided into three groups. They are cited in approximately chronological order, and the first group of six, in prose, opens with the important “Upanisad of the Great Āraṇyaka,” followed by the “Chāndogya.”
Probably contemporary with it are the Taittiriya-Upanisad, the Aitareya-Upanisad, the Kausitaki-Upanisad, and the Kena-Upanisad. The second group comprises five U. in verse, among which the first, the Katha-Upanisad, is notable for its dramatic introductory episode. These are followed by the Iśa-Upanisad, the Svetāśvatara-Upanisad, the Munda-Upanisad, and the Mahānārāyaṇa-Upanisad. Closing the series are three other U. in prose, likewise attributed to specific Vedic schools, but evidently later because they draw extensively from the preceding ones and are more refined in language and style. They bear the titles of Praśna-Upanisad, Maitrāyaṇīya-Upanisad, and Māṇḍūkya-Upanisad. These fourteen U., while expounding a doctrine not yet reduced to a system and hence not without discrepancies and contradictions, all reflect the original cosmic conception inappropriately designated by the Western term pantheism, but better defined as theopanism, the doctrine of the God-All. This culminates in idealistic monism, which may be summarized in these three apothegms of the Brahman Yājñavalkya: 1° Atman is the subject of knowledge. Bṛhadāraṇyaka-Upanisad, III, 8, 11: “Truly, O Gārgī, this Imperishable is the Seer unseen, the Hearer unheard, the Thinker unthought, the Knower unknown; there is no other seer than it, no other hearer than it, no other thinker than it, no other knower than II. Truly, in this Imperishable are woven and interwoven earth and sky.” 2° Atman as the subject of knowledge is itself unknowable: ibid., II, 4, 14: “How can one know him by whom all knowledge is known? How can one know the knower?” 3° Atman is the sole reality; in it are interwoven and interlaced earth and sky with all that they contain: ibid., IV, 3, 23: “There is no second beyond him, no thing different from him that can be seen.”