Tathwamasi is sanskrit phrase, translated variously as “That art thou,” “That thou art,” “Thou art that“, “You are that” or “That you are“. It is one of the Mahāvākyas in Vedantic Sanatana Dharma . It occurs in Chandogya Upanishad 6.8.7 & in Ganesh Atharvasish.
The meaning of this saying is that the, “Self – in its original, pure, primordial state is wholly or partially identifiable or identical with the ultimate reality”. That is the ground and origin of all phenomena.