Laymen may assume that there are contradictions within the Vedas. Some mantra says that the universe came out of Brahma. Some other mantra says that the universe came out of Vishnu. Upanishads iron out all such differences and establish that they mean one and the same. Upanishads reveal the essence of Vedic knowledge. Upanishads are based on the Vedic concept of Brahman which is the one and only supreme truth.