“That art thou” (tat tvam asi), this word of the old Brāhman Āruṇi to his son, which became the “great formula” (mahāvākya) of Vedāntic truth, reduced the entire spectacle of nature to its single, all-pervading, most subtle, absolutely intangible, hidden essence.
Zimmer establishes tat tvam asi as the mahāvākya of Vedānta, tracing its origin to the Chāndogya Upaniṣad and identifying it as the formula that collapses all phenomenal multiplicity into the singular, hidden Ātman-Brahman identity.
, Philosophies of India, 1951thesis