The philosophical foundation of Hinduism — transmitted orally from teacher to student for centuries before being written down. The Bhagavad Gita is the Upanishads applied to action and devotion; the Upanishads are the source. Their central claim: the self at your core (Atman) is identical with the ground of all existence (Brahman). Everything else in the tradition is commentary on that single recognition.
Recommended translation: Patrick Olivelle (Oxford) for scholarly accuracy. Eknath Easwaran for readability.
The Four Great Sayings — Mahavakyas
Prajnanam Brahma
Consciousness is Brahman
Aitareya Upanishad · Rig Veda
Aham Brahmasmi
I am Brahman
Brihadaranyaka Upanishad · Yajur Veda
Tat Tvam Asi
That thou art
Chandogya Upanishad · Sama Veda
Ayam Atma Brahma
This Self is Brahman
Mandukya Upanishad · Atharva Veda
Select an Upanishad
Where to Start
If you've read the Bhagavad Gita: start with the Chandogya (Tat Tvam Asi) or the Katha (the chariot analogy — you'll immediately recognize its echo in the Gita's opening scene). Then the Mandukya — twelve verses, the whole teaching.
If you're coming fresh: the Katha is the most narrative and accessible — it reads like a story before it becomes philosophy. Then the Isha (eighteen verses, the ethics of someone who knows Brahman). Then the Chandogya for the core identity claim.
ॐ
"That which cannot be seen by the eye, but by which the eye sees — know that alone as Brahman." — Kena Upanishad 1.6
Non-denominational reference guide · Patrick Olivelle or Eknath Easwaran translations recommended