 Paul Deussen states that the concept of "Brahman" in the Upanishads expands to metaphysical, ontological and soteriological themes, such as it being the "primordial reality that creates, maintains and withdraws within it the universe", the "principle of the world", the "absolute", the "general, universal", the "cosmic principle", the "ultimate that is the cause of everything including all gods", the "divine being, Lord, distinct God, or God within oneself", the "knowledge", the "soul, sense of self of each human being that is fearless, luminuous, exalted and blissful", the "essence of liberation, of spiritual freedom", the "universe within each living being and the universe outside", the "essence and everything innate in all that exists inside, outside and everywhere".Gavin Flood summarizes the concept of "Brahman" in the Upanishads to be the "essence, the smallest particle of the cosmos and the infinite universe", the "essence of all things which cannot be seen, though it can be experienced", the "self, soul within each person, each being", the "truth", the "reality", the "absolute", the "bliss" ("ananda").According to Radhakrishnan, the sages of the Upanishads teach Brahman as the ultimate essence of material phenomena that cannot be seen or heard, but whose nature can be known through the development of self-knowledge ("atma jnana").