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 book of one question: "by whom?" — by whom the mind is moved, by what the breath is sent, by whose power the eye sees. The answer is that He is the ear of the ear and the mind of the mind, not one more object among others; and the parable of the gods grown proud of victory shows whose power it was.