Vedanta has been interpreted as the 'last chapters, parts of the ' and alternatively as 'object, the highest purpose of the Veda'. The concepts of (ultimate reality) and (soul, self) are central ideas in all of the Upanishads, and 'know that you are the Ātman' is their thematic focus. Along with the and the, the mukhya Upanishads (known collectively as the ) provide a foundation for the several later schools of, among them, two influential schools of Hinduism. More than 200 Upanishads are known, of which the first dozen or so are the oldest and most important and are referred to as the principal or main ( ) Upanishads. The mukhya Upanishads are found mostly in the concluding part of the and and were, for centuries, memorized by each generation and passed down.