A class of scripture in Hinduism, the Upaniṣads were first written down in the sixth century CE. Thirteen Upaniṣads form part of the sacred Vedas, although well over 100 Upaniṣads are known. Chiefly philosophical works on human existence and the universe, the Upaniṣads discuss topics such as the nature of reality and the definition of and path to the soul’s liberation. They form the basis of key concepts in Hinduism, Buddhism and Jainism, and thus are key to the development of Indian culture.