Chandogya Upanishad — The Teaching of Uddalaka
The Chandogya Upanishad contains the Tat Tvam Asi teaching and the famous teaching of the salt dissolved in water — the most sustained and systematic account of Brahman-Atman identity in the Upanishads.
The Chandogya Upanishad is one of the most important of all Upanishads — its sixth chapter, the teaching of Uddalaka Aruni to his son Shvetaketu, contains the most systematic and pedagogically developed account of Brahman-Atman identity. The famous nine-fold repetition of 'Tat Tvam Asi' (That thou art) is introduced each time through a different analogy: the salt in water (which cannot be seen but is tasted everywhere); the nyagrodha (fig) tree potential in its tiny seed; the bees making honey from many flowers; and others. Each illustration establishes the same point: the 'subtle essence' that pervades all things is identical with the Atman of the student. The text also contains: the AUM teaching (3.14.1 — 'All this is AUM'); the Vaishvanara Atman teaching (identification of Brahman with cosmic fire); and the Akashi Brahman teaching (Brahman as infinite space). Together these make the Chandogya one of the most complete Upanishadic accounts of the Brahman-Atman identity.
As the rivers flowing east and west merge in the sea, forget their name and form, and become one with the ocean — so too do all beings merge in the supreme Being, forget their name and form.
Comparative philosopher Ninian Smart identified the 'mystical dimension' of religion — direct experience of unity with the ultimate — as present in all major traditions, and pointed to the Chandogya's teaching as the clearest ancient articulation of this universal experience.
Chandogya Upanishad