The schools of Hindu thought crystallized in the Sutra period (400 bc– ad 500). As usually counted there are six orthodox schools accepting the authority of the Vedas, which can be divided into three pairs. These are the Nyaya and Vaisheshika; Samkhya and Yoga; Mimamsa and Vedanta. In each case the first member of the pair may be regarded as a codification or methodology for the metaphysical system represented by the second. Characteristic doctrines are described under the separate headings. There are three unorthodox schools: Buddhism, Jainism, and Carvaka.