To denote that Brahman and Atman are One, the Chāndogya Upanishad uses the term Tat tvam asi, meaning ‘Thou art That’. In modern times, Nisargadatta Maharaj reiterated this fundamental truth of human existence in I Am That, which has been described by Vijai Shankar, an Advaita sage and former medical practitioner, as the only spiritual book you need to read.

In Panosophy, in conformity with the Hidden Harmony, Tathatā is the dual of Śūnyatā, the central notions of ‘Suchness’ and ‘Void’ in Mahāyāna and Hīnayāna Buddhism, as ‘Great Vehicle’ and ‘Small Vehicle’, respectively.