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.
A commentary on the Puruṣa-sūkta, the famous Ṛgvedic hymn on the Puruṣa from whom the world is made. It explains why the hymn is repeated, and says that this knowledge is not given to just anyone, nor for free.