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.
One of the oldest and largest: it opens with the story of a king who went to the forest in disgust at the body, and passes into a teaching on two "selves" — the one that tastes, and the one that watches. Here too is one of the earliest descriptions of the six limbs of yoga.