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.
On how it all began: the Ātman is one, and He creates the worlds, forms the person, and enters him — and the person forgets who is within him. It ends with a saying that became one of the great utterances: "knowledge is Brahman."