The earliest and most ancient layer of Yajurveda samhita includes about 1,875 verses, that are distinct yet borrow and build upon the foundation of verses in Rigveda. The middle layer includes the Satapatha Brahmana, one of the largest Brahmana texts in the Vedic collection. The youngest layer of Yajurveda text includes the largest collection of primary Upanishads, influential to various schools of Hindu philosophy. These include the Brihadaranyaka Upanishad, the Isha Upanishad, the Taittiriya Upanishad, the Katha Upanishad, the Shvetashvatara Upanishad and the Maitri Upanishad.