Hindu philosophy began in the period of the Upanishads (900–500 B.C.), but systematic philosophical elaboration did not appear until several centuries later. Philosophical tenets were presented in the form of aphorisms or sutras, intended to serve as an aid to memory and a basis for oral elaboration. Their extreme conciseness presupposes an oral or written commentary, and the traditions developed through successive layers of commentarial tradition. Although all six schools of classical Hindu philosophy accepted the authority of the Veda, they had widely differing philosophical positions; they developed in competition not only with one another, but also with the so-called heterodox schools, which rejected the authority of the Veda: Buddhism, Jainism, the Ajivikas or skeptics, and the materialist Carvaka school.