Āstika and nāstika are concepts that have been used to classify Indian philosophies by modern scholars, 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

In current Indian languages like Telugu, Hindi and Bengali, āstika and its derivatives usually mean ’theist’, and nāstika and its derivatives denote an ‘atheist’; however, the two terms in ancient- and medieval-era Sanskrit literature do not refer to ’theism’ or ‘atheism’

The terms are used differently in Hindu philosophy

For example, Sāṃkhya is both a non-theistic (as it does not explicitly affirm the existence of God in its classical formulation) and āstika (Vedic) 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 Vishnu in some Hindu traditions

Āstika (Sanskrit: आस्तिक; from Sanskrit: asti, ’there is, there exists’) means one who believes in the existence of a Self or Brahman, etc

It has been defined in one of three ways: as those who accept the epistemic authority of the Vedas; as those who accept the existence of ātman; as those who accept the existence of Ishvara

Nāstika (Sanskrit: na, ’not’ + āstika), by contrast, are those who deny all the respective definitions of āstika; they do not believe in the existence of Self

The six most studied Āstika schools of Indian philosophies, sometimes referred to as orthodox schools, are Nyāyá, Vaiśeṣika, Sāṃkhya, Yoga, Mīmāṃsā, and Vedānta

The four most studied Nāstika schools of Indian philosophies, sometimes referred to as heterodox schools, are Buddhism, Jainism, Chārvāka, and Ājīvika

However, this orthodox-heterodox terminology is a construct of Western languages, and lacks scholarly roots in Sanskrit

Recent scholarly studies state that there have been various heresiological translations of Āstika and Nāstika in 20th century literature on Indian philosophies, but many are unsophisticated and flawed