The Upaniṣads — the philosophical core of the Vedic tradition, revealing the nature of Brahman, the Ātman, knowledge, action and liberation. The ten principal Upaniṣads together with select minor Vaiṣṇava Upaniṣads.
Twelve lines on the syllable Oṃ and the four states: waking, dreaming, deep sleep, and the fourth — that which neither sleeps nor wakes. The most compressed book of the collection: whole treatises are written on it, yet it can be read in a minute.