It is true, no doubt, that the stratum of literature which contains the Upanishads is later than the Samhitâs, and later than the Brâhmanas, but the first germs of Upanishad doctrines go back at least as far as the Mantra period, which provisionally has been fixed between 1000 and 800 b.c. Conceptions corresponding to the general teaching of the Upanishads occur in certain hymns of the Rig-veda-samhitâ, they must have existed therefore before that collection was finally closed. One hymn in the Samhitâ of the Rig-veda (I, 191) was designated by Kâtyâyana, the author of the Sarvânukramanikâ, as an Upanishad. Here, however, upanishad means rather a secret charm than a philosophical doctrine. Verses of the hymns have often been incorporated in the Upanishads, and among the Oupnekhats translated into Persian by Dârâ Shukoh we actually find the Purusha-sûkta, the 90th hymn of the tenth book of the Rig-veda[1], forming the greater portion of the Bark'heh Soukt. In the Samhitâ of the Yagur-veda, however, in the Vâgasaneyisâkhâ, we meet with a real Upanishad, the famous Îsâ or Îsâvâsya-upanishad, while the Sivasamkalpa, too, forms part of its thirty-fourth book[2]. In the Brâhmanas several Upanishads occur, even in portions which are not classed as Âranyakas, as, for instance, the well-known Kena or Talavakâra-upanishad. The recognised place, however, for the ancient Upanishads is in the Âranyakas, or forest-books, which, as a rule, form an appendix to the Brâhmanas, but are sometimes included also under the general name of Brâhmana. Brâhmana, in fact, meaning originally the sayings of Brahmans, whether in the general sense of priests, or in the more special of Brahman-priest, is a name applicable not only to the books, properly so called, but to all old prose traditions, whether contained in the Samhitâs, such as the Taittirîya-samhitâ, the Brâhmanas, the Âranyakas, the Upanishads, and even, in certain cases, in the Sûtras. We shall see in the introduction to the Aitareya-âranyaka, that that Âranyaka is in the beginning
