Adi Shankara learned the sacred texts of Hinduism, like Vedas and Upanishads under his teacher Govinda Bhagavadpada and later wrote extensive commentaries of Hindu sacred texts called Upanishads. In these commentaries, he proposed the theory of Advaita, saying that the Upanishad actually teach that the individual soul (called Atman) is not different from Ultimate Reality (called Brahman). He also taught that there is only one essential principle called Brahman and everything else is a kind of expression of that one Brahman. Because of this theory of one being, his teachings became popular as the "Advaita" ("a" = not, "dvaita" = two, means no-two or non-dual). The way he said this to people was ""Atman is Brahman.""