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.
Here two kinds of knowledge are distinguished for the first time: the lower — all the Vedas and sciences — and the higher, by which the Imperishable is known. Hence its famous images: two birds on one tree, the bow and arrow where the person himself becomes the arrow, and the warning about the "leaky boats" of rites.