The Upanishads have always been regarded in India as the crown of the Veda and as the end of the Veda as implied by the term vedanta. The major Upanishads are not separate books, but the last parts or the penultimate parts of the corresponding Brahmana books. If the Brahmana has an Aranyaka attached to it, then the corresponding Upanishad is at the end of the Aranyaka book. Shatapatha Brahmana has no Aranyaka; thus its last book or chapter contains the Upanishad, the famous Brihadaranyaka Upanishad.
«The Upanishads are not a revolutionary departure from the vedic mind and its temperament and fundamental ideas, but a continuation and development and to a certain extent an enlarging transformation in the sense of bringing out the into open expression all that was held covered in the symbolic vedic speech as a mystery and a secret» [Sri Aurobindo, The Foundations of Indian Culture]. «The rishis of the Upanishads sought to recover the lost or waning knowledge of the Veda by meditation and spiritual experience and they used the text of the ancient mantras as a prop and authority for their own intuitions and perception or else the vedic word was a seed of thought and vision by which they recovered old truths in new forms. What they found, they expressed in other terms more intelligible to the age in which they lived» (Sri Aurobindo, The Secret of the Veda). As examples of the first method, we may mention the Chhandogya Upanishad (3.12.5) or Brhadaranyaka Upanishad (4.4.23) which state, “as stated in the rik, the mantra of Rigveda”. As an example of the second, we may mention the concept of Brahman. RigVeda mentions repeatedly the Supreme One, or the One Truth, ekam sat, which underlines all existence. The Upanishads developed this seed of thought into the magnificent conception of Brahman.
Chhandogya Upanishad (7.7.1) declares that Rig Veda can be understood only by meditation vijnanena. The extensive connection between the Veda Samhitas and Upanishads mentioned in this essay should make a reader sceptical of statements such as, ‘Upanishads are a protest against the externality of vedic practices’.
The Upanishads can be divided into roughly two categories for study. In the first are the metrical Upanishads, relatively smaller in size, such as Isha, Katha, Mundaka, Shvetashvatara, Taittiriya, the middle two chapters of the non-metrical Brhadaranyaka Upanishad etc. These Upanishads use symbolism sparingly, so that one can get some idea of their contents by one’s rational intellect. In the second group belong all other Upanishads which are non-metrical and use symbolism extensively. Both Aitareya Upanishad (1.3.14) and Brhadaranyaka Upanishad (4.2.2) declare that, “Gods love indirect reference or symbolism”, paroksha priya hi devah. The purport is that the cosmic powers devah prefer that the profound truths be expressed only in a symbolic manner so that only the eager and persistent student can understand them. The Brhadaranyaka Upanishad begins with the detailed symbolism of the sacrificial horse, each one of whose limbs such as the face, belly, legs etc., corresponds to a specific cosmic power. For instance Usha, the dawn is the head of the sacrificial horse. The great commentator Shamkara gives a brief explanation without clarifying the deeper meaning. The fifth and the sixth chapters of the Brhadaranyaka Upanishad have verses which for a gross mind appear to be ritualistic recipes for obtaining children of specific characteristics. The commentators practically ignore the fifth and sixth chapters, declaring that they are meant for the householders who deal with mundane matters. Persons who have attempted to pierce the symbolism in the Veda Samhitas can easily get clues to understanding the deeper meanings of these verses behind their apparently gross outward form.
The Upanishads, besides delineating various spiritual experiences, also give a few hints on sadhana, i.e., paths of spiritual realization. These methods of sadhana are called vidyas. The Upanishad does not give much detail about the vidyas because such details cannot be conveyed in print. Typically, a teacher transmits these truths to the students, often in silence. There are not many books in print which discuss the relevance of Upanishadic thought for spiritual practice. One such book is the Light on the Upanishads by Kapali Sastry which discusses some of the secret vidyas in the Upanishads and relates them to the corresponding ideas in Rig Veda. These vidyas are identified either by the name of the teacher like Shandilya vidya, Bhrgu Varuni vidya or by their contents like the Bhuma vidya, Vaishvanara vidya, Madhu vidya, Prana vidya and so on. Vaishvanara means the divine force, which permeates every aspect of existence. This knowledge in the Chandogya Upanishad is related to the similar idea in the Rig Veda contained in the ten hymns to the deity Vaishvanara. The Madhu vidya or the doctrine of the honey or bliss as the foundation of all existence, discussed in detail in Brhadaranyaka Upanishad (2.5) is related to the corresponding mantras in the first book of Rig Veda. Again the particular method of realisation described in the sixth book of Brhadaranyaka Upanishad (6.3.6) involves the chanting of the famous gayatri hymn of Rig Veda (III.62.10) and the madhu mantras in RigVeda (I.90). This particular method is extolled in the Brhadaranyaka Upanishad (6.3.7-6.3.12) stating, “if any one sprinkles it on a dry stump, branches would grow and leaves spring forth”.