The authorities basic texts on Vedanta are the Upanishads, Bhagavad-gita and the Brahmassutra. these three texts are together called the 'Prasthanatraya'. The Brahmasutra consists of 555 sutras, which are presented in four chapters. Sri Veda Vyasa,the author of Brahmasutra has systematized the thoughts of Upanisads and resolved their seeming contradiction and conflicting thoughts herein, thus giving us all a crystal clear idea of the entire Vedantika Philosophy. Sri Adi sankara has written a very lucid and thought-provoking commentary on the Brahmasutra. The first four Brahmasutras with the commentary by Sri Adi sankara indeed contain the entire crux of the Advaita Vedanta, which are called the 'Catuh-Sutri'.