The Upanishads contain accounts of the mystic significance of the syllable “Aum”, explanations of mystic words like “Tajjalan”, which are intelligible only to the initiated, and secret texts and esoteric doctrines. Upanishad became a name for a mystery, a secret, “rahasyam” communicated only to the tested few. When the question of man’s final destiny was raised, Yajnavalkya took his pupil aside and whispered to him the truth.
According to the Chhandogya Upanishad, the doctrine of Brahman may be imparted by a father to his elder son or to a trusted pupil, but not to another, whoever he may be, even if the latter should give him the whole earth surrounded by the waters and filled with treasures. In many cases it is said that the teacher communicates the secret knowledge only after repeated entreaty and severe testing.
Samkara derives the word Upanishad as a substantive from the root “sad”, to loosen, to reach, or to destroy with “upa” and “ni” as prefixes and “kvip” as termination. If this derivation is accepted, Upanishad means brahma-knowledge by which ignorance is loosened or destroyed. The treatises that deal with Brahma knowledge are called the Upanishads and so pass for the Vedanta.
The different derivations together make out that the Upanishads give us both spiritual vision and philosophical argument. There is a core of certainty which is essentially incommunicable except by a way of life. It is by a strictly personal effort that one can reach the truth.