Āstika (Sanskrit: आस्तिक, IAST: āstika) and nāstika (Sanskrit: नास्तिक, IAST: nāstika) are mutually exclusive terms that modern scholars use to classify the schools of Indian philosophy as well as some Hindu, Buddhist and Jain texts. The various definitions for āstika and nāstika philosophies have been disputed since ancient times, and there is no consensus. One standard distinction, as within ancient- and medieval-era Sanskrit philosophical literature, is that āstika schools accept the Vedas, the ancient texts of India, as fundamentally authoritative, while the nāstika schools do not. However, a separate way of distinguishing the two terms has evolved in current Indian languages like Telugu, Hindi and Bengali, wherein āstika and its derivatives usually mean 'theist', and nāstika and its derivatives denote 'atheism'.
Still, philosophical tradition maintains the earlier distinction, for example, in identifying the school of Sāṃkhya, which is non-theistic (as it does not explicitly affirm the existence of God in its classical formulation), as āstika (Veda-affirming) philosophy, though "God" is often used as an epithet for consciousness (purusha) within its doctrine. Similarly, though Buddhism is considered to be nāstika, Gautama Buddha is considered an avatar of the god Vishnu in some Hindu denominations. Due to its acceptance of the Vedas, āstika philosophy, in the original sense, is often equivalent to Hindu philosophy: philosophy that developed alongside the Hindu religion.
