The emergence of Vedanta school represented a period when a more knowledge-centered understanding began to emerge. These focussed on jnana (knowledge) driven aspects of the Vedic religion and the Upanishads. This included metaphysical concepts such as ātman and Brahman, and emphasized meditation, self-discipline, self-knowledge and abstract spirituality, rather than ritualism. The Upanishads were variously interpreted by ancient and medieval era Vedanta scholars. Consequently, the Vedanta separated into many sub-schools, ranging from theistic dualism to non-theistic monism, each interpreting the texts in its own way and producing its own series of sub-commentaries.