These books are clearly much later than the Samhitā books. In course of time, the outward rituals became very strong; thus the crust of ritualism almost completely enveloped the deep spiritual knowledge of the mantrās. The rişhis of a much later age (one thousand years or more) attempted to recover the spiritual knowledge independently by means of tapas. The philosophical truths and occult knowledge recovered by the rişhis are contained in the Upanishad books. The special feature is that these books are more easily understood by intellectuals than the Veda Samhitās. The Upanishads occasionally mention the Rig vedic mantrās but to use them is not their main occupation. They are concerned with establishing the supreme truth. And in the line of their endeavour, they may and do refer to these mantrās by way of illustration to find support for their own conclusions, comment on them whenever necessary and make a rightful use of them for purposes of propagation of spiritual knowledge to their disciples and truth seekers.
There is the list of 108 Upanishads compiled in the Muktika Upanishad. We are dealing here only with the famous thirteen Upanishads which are associated with a Brāhmaņa book or Āraņyaka book, typically constituting their ending chapter or chapters. They are famous because the great commentator on Upanishads, Bādarāyaņa quoted only from these 13 Upanishads in his classic sūtra book, “brahma sūtrās”. They are:
īsha, kena, kaţha, prashna, muņdaka, māndūkya, aitareya, taittirīya, chhāndogya, bŗhadāraņyaka, kaushītaki,shvetāshvatara, mahānārāyaņa.
Note that the three Upanishads mundaka, māndūkya and prashna which play a crucial role in the vedāntic interpretations are all associated with the Atharva Veda, showing the sacredness of the fourth Veda also.
The text of all the 13 Upanishads put together is one half or less that of the Rigveda Samhita alone. The two massive Upanishads, Chhāndogya and Bŗhadāraņyaka constitute eighty percent of the text of all the 13 Upanishads.
There are several passages in these two Upanishads which are couched in ritualistic terms and would sound peculiar to the ordinary rationalistic person. We can understand their depth only if we understand the symbolism behind them.
Only if we have a mastery over the symbolism of the mantrās of Rigveda Samhitā, then the inner meaning of these apparently controversial mantrās will become transparent.
Now we will consider the major traditional interpretations of all the Veda books.