In ancient literature, there used to be reference to only three Vedas
. Even Manu has often referred to only these three Vedas. The fourth one, Atharva
, according to some scholars, was a later addition.
Each Veda consists of the following parts- 1. Sanhita (Sacred hymns and prayers). 2. Brahmana- a treatise relating to rituals, prayer and sacrificial ceremonies. This also contains stories about gods and goddesses. 3. Aranyaka (forest texts) are appendices to the Brahmanas. The Upanishads represent the final stage of the tradition of the Vedas. Their teachings form the basis for much of the later Hindu philosophy.
Most of the stories of the Upanishads revolve around the nature of Reality and the concept of a single supreme being. The equation of the Atman (the self) with the Brahman (ultimate reality) is summed up in the phrase "tat tvam asi" (that art thou) in the Chhandogya Upanishad. The stories selected for this volume amply illustrate the fact that the sages in those bygone days were imbued with the spirit of scientific enquiry and there is also implicit acknowledgement of the fact that knowledge is not the monopoly of any select group. For example, in one of the stories, Raikva, a cart driver, is approached with humility by one of the great kings of his time, Janashruti, with the request to impart knowledge about Brahman.
The Upanishads are the basis of the shad-darshanas, the six major systems of Hindu philosophy.