The Upanishads are Hindu scriptures that constitute the core teachings
of Vedanta. They form the Jnana-Kanda or the portions concerning
the knowledge/wisdom of the ultimate principle referred as Brahman.
The term Upanishad derives from upa- (nearby), ni- (at the proper
place, down) and sad, that is "sitting down near" a teacher
in order to receive instruction. The most important upanishads that
are commented by the founders all major schools of 'sanathana dharma'
are Chandogya, Brihadaranyaka, Mandukya, Mundaka, Katha, Kena, Isa,
Aitareya, Taittiriya, Prasna and Svetasvatara Upanishads.
The Upanishads speak of a universal spirit (Brahman) and of an
individual soul (Atman),and assert the identity of both. Brahman
is the ultimate, both transcendent and immanent, the absolute infinite
existence, the sum total of all that ever is, was, or shall be.
The mystical nature and intense philosophical bent of the Upanishads
has led to their explication in numerous manners, giving birth to
three main schools (advaita, visishtadvaita and dwaita) of Vedanta.