UPANIŞAD. — U. means “esoteric doctrine,” secret, to be communicated only to a few initiates. It is the common designation of a series of works differing in age, length, and character, but all equally intended to Brahman (v.), the cosmic principle, and on its identity with the Atman, the “self,” the psychic principle, the soul. Only the U. composed within the Vedic Schools (sākhā), of which they are the final product, are canonical. They are therefore also collectively called Vedānta, “end of the Veda,” a name also given to the monism of Bādarāyaṇa and Śaṅkara, founded on the Upaniṣadic doctrines (v. VEDĀNTA).
Attached to the Brāhmaṇa (v.) by means of the Āraṇyaka (v. BRAHMANESIMO), the U. constitute their epilogue, replacing the allegorical interpretation of the ritual with philosophy. The formation of the new doctrine is usually assigned a period of approximately three centuries (800–500 B.C.), but it is impossible to determine the age to which the individual U. belong. Following Prof. Deussen, those enjoying the reputation of canonical texts and regarded as revelation (truti) are divided into three groups. They are cited in approximately chronological succession, and the first group of six, in prose, opens with the important “U. of the Great Āraṇyaka,” followed by the “Chāndogya-
U.,” probably contemporary, the Taittiriya-U., the Aitareya-U., the Kausitaki-U., and the Kena-U. The second group comprises five U. in verse, among which the first, the Kāthaka-U., stands out for its dramatic introductory episode. Then come the Īśā-U., the Śvetāśvatara-U., the Muṇḍaka-U., and the Mahānārāśyana-U. The series concludes with three other U. in prose, likewise attributed to particular Vedic schools, but evidently later because they draw extensively on the preceding works and are more refined in language and style. They bear the titles Praśna-U., Maitrāśyana-U., and Māṇḍākyā-U. These fourteen U., although presenting a doctrine not yet reduced to a system and consequently not free from discrepancies and contradictions, all reflect the original cosmic conception improperly designated by the Western name of pantheism, but which might more accurately be defined as theopanism, the doctrine of the God-All. This culminates in idealistic monism, which may be summarized in these three apophthegms of the brahman Vāṭāvalkya: 1° The Ātman is the subject of knowledge. Bṛhadāraṇyaka-U., III, 8, 11: “In truth, O Gōrgī, this Imperishable is the Seer who is not seen, the Hearer who is not heard, the Thinker who is not thought, the Knower who is not known; there is no other seer than he, there is no other hearer than he, there is no other thinker than he, there is no other knower than he. In truth, in this Imperishable space is woven and interwoven.” 2° The Ātman, as the subject of knowledge, is itself unknowable: ibid., II, 4, 14: “How can one know him from whom the knowledge of everything proceeds? How can one know the knower?” 3° The Ātman is the sole reality; in him space is woven and interwoven, together with all that is gathered within it: ibid., IV, 3, 23: “There is no second besides him, no thing different from him that could be seen.”