The Upanishads are late Vedic Sanskrit texts of religious teaching and ideas still revered in Hinduism. Of all Vedic literature, the Upanishads alone are widely known, and their central ideas are at the spiritual core of Hinduism. Around 108 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 Kena Upanishad is notable in its discussion of Brahman with attributes and without attributes, and for being a treatise on "purely conceptual knowledge".