In India, Brāhman is the ultimate enigma, – of which the Upaniṣads sparingly reveal some secret teachings.
The word Upaniṣad means several things: the mystery underlying all things; a secret, mysterious, mystical doctrine; the writings relating to Brāhmaṇas, (whose purpose is to expose the secret meaning of the Vedas); the source of the philosophy of Vedānta and Sāṃkhya.
Ṡaṅkara provides a more relaxed explanation of this complicated name: « By adding upa (approach), and ni (deposit) to the SAD root, the meaning is « dissolution » (viṡaraṇa); we have a movement (approach or reach/gati) and a untying (avasādana)… Upaniṣad is the knowledge that has as its object the knowable (vedyavastu). Knowledge is called Upaniṣad by association with its purpose. »
The root SAD, which is the heart of the word, alone has a wide spectrum of meaning: « to sit down (during a sacrifice); to observe carefully; to faint, to collapse from despair, distress, despair, perish; to afflict, ruin, destroy. «
SAD : Resonances of the attitude of the officiant who makes the sacrifice, and the most extreme feelings of the one who despairs, or the one who destroys.
The search for knowledge is not a long, quiet river. Dissolution, untying, distress, despair, destruction, accompany it.
Apart from Upaniṣad, there are many other secrets, for example in Brahmanic singing, where the issue is « what is secret » (guhā). There are also some in speech: « Speech is measured between four quarters as known to Brāhmanes who have intelligence; three hidden are motionless; humans speak a quarter of the speech. » (Ṛg Veda I.164.45)
For each word, three out of four parts remain hidden, motionless. Who hears them?