All the Sastras derive their value and validity from their
source - the Vedas. They lay down modes and norms in consonance
with the principles and purposes defined in the Vedas. To
discriminate between good and bad, the Sastras have to be resorted
to.
The Vedas are A-pourusheya: that is, they have no identifiable
human authors. They have emerged from God Himself and they are
'heard' by sages attuned to the Voice of the Divine. They
communicated the Word to their pupils and they in turn taught them
to their disciples. This process of imparting the Vedas and the
Wisdom enshrined in them has continued through generation after
generation of gurus and disciples up to our own times.
The Upanishads are the very core of the Vedas, the very essence
of their teachings. The Brahma Sutra and the Bhagavad Githa
contain the very essence of the teachings of the Upanishads. These
three scriptural texts are, therefore, designated as the
Prasthaana Thraya, the Three Source Texts. Since these have been
learnt by listening to the Guru, they are, along with the Vedas,
named Sruthi, the "Heard".
Acquisition of the Higher knowledge alone can fulfil the main
purpose of human life. Such knowledge makes one aware that he is
not the inert non-sentient body, etc., but that he is
Consciousness itself manifesting as the embodiment of
Sath-Chith-Ananda, Existence - Awareness - Bliss. When this Truth
dawns and is experienced, man is liberated; he is freed from the
fog of ignorance, Ajana, even while life endures till its term
ends. He becomes a Jeevan-Muktha.