Advaita literally means "not two, sole, unity". It is a sub-school of Vedanta, and asserts spiritual and universal non-dualism. Its metaphysics is a form of absolute monism, that is all ultimate reality is interconnected oneness. This is the oldest and most widely acknowledged Vedantic school. The foundational texts of this school are the Brahma Sutras and the early Upanishads from the 1st millennium BCE. Its first great consolidator was the 8th century scholar Adi Shankara, who continued the line of thought of the Upanishadic teachers, and that of his teacher's teacher Gaudapada. He wrote extensive commentaries on the major Vedantic scriptures and is celebrated as one of the major Hindu philosophers from whose doctrines the main currents of modern Indian thought are derived.