If now we ask what has been thought of the Upanishads by Sanskrit scholars or by Oriental scholars in general, it must be confessed that hitherto they have not received at their hands that treatment which in the eyes of philosophers and theologians they seem so fully to deserve. When the first enthusiasm for such works as Sakuntalâ and Gîta-Govinda had somewhat subsided, and Sanskrit scholars had recognised that a truly scholarlike study of Indian literature must begin with the beginning, the exclusively historical interest prevailed to so large an extent that the hymns of the Veda, the Brâhmanas, and the Sûtras absorbed all interest, while the Upanishads were put aside for a time as of doubtful antiquity, and therefore of minor importance.
My real love for Sanskrit literature was first kindled by the Upanishads. It was in the year 1844, when attending Schelling's lectures at Berlin, that my attention was drawn to those ancient theosophic treatises, and I still possess my collations of the Sanskrit MSS. which had then just arrived at Berlin, the Chambers collection, and my copies of commentaries, and commentaries on commentaries, which I made at that time. Some of my translations which I left with Schelling, I have never been able to recover, though to judge from others which I still possess, the loss of them is of small consequence. Soon after leaving Berlin, when continuing my Sanskrit studies at Paris under Burnouf, I put aside the Upanishads, convinced that for a true appreciation of them it was necessary to study, first of all, the earlier periods of Vedic literature, as represented by the hymns and the Brâhmanas of the Vedas.
In returning, after more than thirty years, to these favourite studies, I find that my interest in them, though it has changed in character, has by no means diminished.
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, 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. 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 a Brâhmana, a mere continuation of the Aitareya-brâhmana, explaining the Mahâvrata ceremony, while its last book contains the Sûtras or short technical rules explaining the same ceremony which in the first book had been treated in the style peculiar to the Brâhmanas. In the same Aitareya-âranyaka, III, 2, 6, 6, a passage of the Upanishad is spoken of as a Brâhmana, possibly as something like a Brâhmana, while something very like an Upanishad occurs in the Âpastamba-sûtras, and might be quoted therefore as a Sûtra. At all events the Upanishads, like the Âranyakas, belong to what Hindu theologians call Sruti, or revealed literature, in opposition to Smriti, or traditional literature, which is supposed to be founded on the former, and allowed to claim a secondary authority only; and the earliest of these philosophical treatises will always, I believe, maintain a place in the literature of the world, among the most astounding productions of the human mind in any age and in any country.