The Upanishads (the principal Upanishads)

The Upanishads are the concluding portions of the Vedic literature, prose and verse texts later treated as the foundation of Vedanta. The oldest, the Bṛhadāraṇyaka and Chāndogya, are largely dialogues and teachings rather than systematic treatises. Their recurring question is what underlies everything, and how that ground (brahman) relates to the self (ātman). In the Chāndogya, Uddālaka Āruṇi teaches his son Śvetaketu with salt dissolved in water and the refrain 'you are that'. In the Bṛhadāraṇyaka, Yājñavalkya describes the self as the unseen seer that can only be indicated by 'not this, not that', and discusses immortality with his wife Maitreyī. Later Upanishads include the Kaṭha, where the boy Naciketas questions Death, and the very short Māṇḍūkya on waking, dream, deep sleep and a fourth state. Later schools read them as teaching non-dualism, qualified non-dualism or dualism.

They are among the oldest philosophical texts in the world, and every school of Vedanta, from Śaṅkara to Madhva, is built as a reading of them.