Brihadaranyaka Upanishad — The Great Forest Teaching
The Brihadaranyaka Upanishad is the longest and oldest of the major Upanishads — a vast philosophical treatise containing the most direct teachings on Brahman-Atman identity.
The Brihadaranyaka Upanishad (the 'great forest teaching' — Brihad = great, Aranya = forest, relating to teachings given in forest retreats) is the longest and among the oldest of the major Upanishads (c. 700-600 BCE). Its philosophical contributions are unmatched: (1) The Yajnavalkya-Maitreyi dialogue on the nature of the Atman and its relationship to Brahman; (2) The Yajnavalkya-Gargi debate — one of the earliest philosophical dialogues on the nature of ultimate reality; (3) The declaration 'Neti neti' as the method of approaching Brahman; (4) The Honey Doctrine (Madhu Vidya) — all beings are honey (nourishment) for each other, and Brahman is the honey of all; (5) The Antaryamin Brahmana — Brahman as the inner controller of all. Shankara's commentary on the Brihadaranyaka is his most extensive and is considered the foundation of all subsequent Advaita commentary.
That which is not expressed by speech, but by which speech is expressed — know that alone as Brahman.
Philosopher Paul Deussen — Schopenhauer's student and the first Western scholar to systematically translate and analyze the Upanishads — declared the Brihadaranyaka 'the most ancient philosophic work of world literature,' noting that its conception of Brahman-Atman identity was reached 1,500 years before the birth of philosophy in Greece.
Kena Upanishad