 Gavin Flood states that the Vedic era witnessed a process of abstraction, where the concept of "Brahman" evolved and expanded from the power of sound, words and rituals to the "essence of the universe", the "deeper foundation of all phenomena", the "essence of the self (Atman, soul)", and the deeper "truth of a person beyond apparent difference".The primary focus on the early Upanishads is "Brahmavidya" and "Atmavidya", that is the knowledge of Brahman and the knowledge of Atman (self, soul), what it is and how it is understood.The texts do not present a single unified theory, rather they present a variety of themes with multiple possible interpretations, which flowered in post-Vedic era as premises for the diverse schools of Hinduism.