The basic doctrine of Advaita Vedanta is found in the Upanishads in the Vedas and is that atman or Brahman is the only reality which is one without a second (Chandogya Upanishad VI 2.1). Upanishads are the Shruti Prasthana of Upanishads. Monism is one of the strands in the Upanishads.
The word Brahman is derived from the root ‘brih’ which means
to grow or to evolve. Chandogya Upanishad has described Brahman as tajjalan –
tat (that) from which the universe arises, ja into which it returns, and la, by
which it is sustained and lives.
Taittiriya Upanishad defines Brahman as that from which all
these things are born, by which they live and into which they merge.
Tatittiriya Upanishad describes the evolution of five sheaths
(koshas) in the following order – it begins with annamaya kosha (matter), which
is unconscious and without life. From matter emerges pranamaya kosha (life).
From life emerges manomaya kosha (mind) and mind causes vijnanamaya kosha (the
emergence of reason) and then follows anandamaya kosha (the blissful state) and
ultimately, Ananda (absolute bliss) itself is reached. This is the highest
level, the level of the mystic, wherein the triad of knower, known, and knowledge
is fused into the transcendental unity of the Absolute Brahman.
Brahman is the highest reality because it transcends all. Yet
it underlies as their locus. It is sarva-bhutantaratma (the self of all) and it
is antaryamin (the inner controller of all). As all the spokes are contained in
the axle and the wheel, so all beings, all devas, all worlds, all organs are
contained in the Supreme Self, Brahman. (Brihadaranyaka Upanishad).
Katha Upanishad describes Brahman as self-luminous, immortal,
the support of all worlds, the highest, etc.
Chandogya Upanishad holds that in the beginning there was
being alone, one without a second. Svetasvatara Upanishad informs that he is
the beginning, the source of the causes which unite (the self with the body).
He is to be seen as beyond the three kinds of time (past, present and future) and
as without parts.
According to Katha Upanishad, one who sees difference and
duality here in this world, remains under bondage, but one who sees non-duality
is liberated (2.1.11).
In this connection the dialogue between Svetaketu and Aruni
Uddalaka and Uddalaka’s dictum, by which the inaudible becomes heard, the
imperceptible becomes perceived, the unknowable becomes known, are very meaningful
(Chandogya Upanishad 6.1.2.7). This dialogue asserts in the mahavakya, the
grand sentence ‘tat tvam asi’ (That thou art).
Brahman has often been described through negation as neti,
neti (Brihadaranyaka Upanishad 4.2.4, 4.422, 4.5.15), the method of defining categories
that cannot be unambiguously affirmed.
The dialogue between Yajnavalkya and Maitreyi in
Brihadaranyaka Upanishad underlines the central theme of Advaitic wisdom, that
atmanastu kamaya sarvam priyam bhavati – anything becomes desirable, or
valuable simply for the sake of the self. Thus atman is of the highest value,
so it should be listened to, reflected upon, concentrated on, and realized.