“This whole world has that as its soul; that is Reality; that is Ātman; that art thou, Śvetaketu.” ‘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 identifies the Atman doctrine’s culmination in the Chandogya Upanishad’s tat tvam asi formula, which collapses all phenomenal multiplicity into a single hidden essence.
, Philosophies of India, 1951thesis