Āstika (, IAST: ) and Nāstika (, IAST: ) are mutually exclusive terms that modern scholars use to classify the schools of Indian philosophy as well as some Hindu, Buddhist and Jain texts.Perrett, Roy. 2000. Indian Philosophy. Routledge. . p. 88.Mittal, Sushil, and Gene Thursby. 2004. The Hindu World. Routledge. . pp. 729–30. The various definitions for āstika and nāstika philosophies have been disputed since ancient times, and there is no consensus.Nicholson, Andrew J. 2013. Unifying Hinduism: Philosophy and Identity in Indian Intellectual History. Columbia University Press. . ch. 9.Doniger, Wendy. 2014. On Hinduism. Oxford University Press. . p. 46. 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.Grayling, A. C. (2019). The History of Philosophy. Penguin Books. p. 519.Chatterjee, Satischandra, and Dhirendramohan Datta. 1984. An Introduction to Indian Philosophy (8th reprint ed.). Calcutta: University of Calcutta. p. 5n1:

"In modern Indian languages, 'āstika' and 'nāstika' generally mean 'theist' and 'atheist,' respectively. But in Sanskrit philosophical literature, 'āstika' means 'one who believes in the authority of the Vedas'. ('nāstika' means the opposite of these). The word is used here in the first sense. The six orthodox schools are 'āstika', and the Cārvāka is 'nāstika' in both the senses." 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'.For instance, the Atheist Society of India produces a monthly publications Nastika Yuga, which it translates as 'The Age of 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. "By reasoning, the material principle itself simply evolves into complex forms, and there is no need to hold that some spiritual power governs the material principle or its ultimate source." Similarly, though Buddhism is considered to be heterodox nāstika, Gautama Buddha is considered an avatar of the god Vishnu in some Hindu denominations.Literature review of secondary references of Buddha as Dashavatara which regard Buddha to be part of the standard list: Britannica A Dictionary of Asian Mythology By David Adams Leeming p. 19 "Avatar" Hinduism: An Alphabetical Guide By Roshen Dalal p. 112 "Dashavatara" ""The standard and most accepted list found in Puranas and other texts is: ... Rama, Krishna, Buddha, Kalki." The Illustrated Encyclopedia of Hinduism: A-M p. 73 "Avatar" Hindu Gods and Goddesses By Sunita Pant Bansal p. 27 "Vishnu Dashavatara" Hindu Myths (Penguin Books) pp. 62–63 The Book of Vishnu (see index) Seven secrets of Vishnu By Devdutt Pattanaik p. 203 "In the more popular list of ten avatars of Vishnu, the ninth avatar is shown as Buddha, not Balarama." A Dictionary of Hinduism p. 47 "Avatara" BBC 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.

Āstika (; from Sanskrit: , '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:GS Ghurye, Indian Sociology Through Ghurye, a Dictionary, Ed: S. Devadas Pillai (2011), , page 354 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 (; from Sanskrit: , 'not' + ), 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 five most studied Nāstika schools of Indian philosophies, sometimes referred to as heterodox schools, are Buddhism, Jainism, Chārvāka, Ājīvika, and Ajñana.For an overview of this method of classification, with detail on the grouping of schools, see: 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.

Etymology Āstika is a Sanskrit adjective and noun that derives from asti ('there is or exists'), meaning 'knowing that which exists' or 'pious.' The word Nāstika (na, not, + ) is its negative.

One of the traditional etymologies of the term āstika—based on Pāṇini's Aṣṭādhyāyī 4.4.60 ("astināstidiṣṭam matiḥ")—defines the concept as 'he whose opinion is that Īśvara exists' (asti īśvara iti matir yasya). According to Sanskrit grammarian Hemachandra, āstika is a synonym for 'he who believes'. Other definitions include: 'opposite of nāstika' (nāstika bhinna); 'he whose idea is that Īśvara exists' (īśvara asti iti vādī); and 'he who considers the Vedas as authorities' (vedaprāmāṇyavādī).

As used in Hindu philosophy, the differentiation between āstika and nāstika does not refer to theism or atheism. The terms often, but not always, relate to accepting Vedic literature as an authority, particularly on their teachings on Self. The Veda and Hinduism do not subscribe to or include the concept of an almighty that is separate from oneself i.e. there is no concept of God in the Christian or Islamic sense. N. N. Bhattacharya writes:

Āstika is also a name, such as that of a Vedic scholar born to the goddess Mānasā ('Mind') and the sage Jaratkaru.George Williams (2003), Handbook of Hindu Mythology, Oxford University Press, , page 65

Classification of schools

The terms Āstika and Nāstika have been used to classify various Indian intellectual traditions.

Āstika The āstika schools are six systems or ṣaḍdarśana that consider the Vedas a reliable and authoritative source of knowledge. These are often coupled into three groups for both historical and conceptual reasons.

Nyāyá-Vaiśeṣika Nyaya, the school of logic Vaisheshika, the atomist school Sāṃkhya-Yoga Samkhya, the enumeration school Yoga, the school of Patañjali (which assumes the metaphysics of Sāṃkhya) Mimāṃsā-Vedanta Mīmāṃsā, the tradition of Vedic exegesis Vedanta or Uttara Mimāṃsā, the Upaniṣadic tradition.

Nāstika

The main schools of Indian philosophy that reject the Vedas were regarded as heterodox in the tradition:

Charvaka Ājīvika Ajñana

Many scholars also use the term nāstika in the context of heterodoxy for: Buddhism and Jainism in India, though some scholars have argued against this view.

Many scholars regard them as heterodox (nāstika), explained by Gavin Flood as "At an early period, during the formation of the Upaniṣads and the rise of Buddhism and Jainism, we must envisage a common heritage of meditation and mental discipline practiced by renouncers with varying affiliations to non-orthodox (Veda-rejecting) and orthodox (Veda-accepting) traditions. ... These schools [such as Buddhism and Jainism] are understandably regarded as heterodox (nāstika) by orthodox (āstika) Brahmanism."

However, some other scholars argue against classifying Buddhism and Jainism as heterodox nāstika, such as Nicholson, who argues that "āstika" and "nāstika" taxonomy developed within particular historical and doxographical contexts, and pre-modern Indian thinkers often viewed philosophical traditions through more complex frameworks than the modern six-orthodox versus three-heterodox scheme, and therefore, Buddhism should not be clubbed together with other nāstika philosophies, such as Charvaka.

On a similar note, Halbfass argues that āstika and nāstika labels were employed from Brahmanical perspectives and reflect sectarian and polemical classifications rather than neutral descriptions of Indian intellectual traditions, further noting that Buddhism possesses sophisticated metaphysical, epistemological, and ethical systems and cannot simply be reduced to "heterodox" from a philosophical standpoint. Another scholar, Richard King, argues that Buddhism should be studied as one of the major Indian philosophical traditions rather than merely as a "heterodox" rejection of Vedic orthodoxy, and criticizes several aspects of the orthodox/heterodox framework.

Tantric traditions in Hinduism have both āstika and nāstika lines; as Banerji writes in Tantra in Bengal:

Usage in religion

Hinduism

Manusmriti, in verse 2.11, defines Nāstika as those who do not accept "Vedic literature in entirety based on two roots of science of reasoning (Śruti and Smriti)". The ninth-century Indian scholar Medhatithi analyzed this definition and stated that Nāstika does not mean someone who says "Vedic literature is untrue", but rather one who says "Vedic literature is immoral". Medhatithi further noted verse 8.309 of Manusmriti, to provide another aspect of the definition of Nāstika as one who believes, "[t]here is no other world, there is no purpose in giving charity, there is no purpose in rituals and the teachings in the Vedic literature."

Manusmriti does not define, or imply a definition for Āstika. It is also silent or contradictory on specific rituals such as animal sacrifices, asserting Ahimsa (non-violence, non-injury) is dharma in its verses such as verse 10.63 based on Upanishadic layer of Vedic literature, even though the older layer of Vedic literature mention such sacrifices unlike the later layer of Vedic literature.Sanskrit: Manusmriti with six scholar commentaries VN Mandlik, page 1310English: Manusmriti 10.63 Berkeley Center for World Religion, Peace and World Affairs, Georgetown University Indian scholars, such as those from Samkhya, Yoga, Nyaya and Vedanta schools, accepted Āstika to be those that include Śabda (; or Aptavacana, testimony of Vedic literature and reliable experts) as a reliable means of epistemology, but they accepted the later ancient layer of the Vedic literature to be superseding the earlier ancient layer.

Without reference to Vedas

In contrast to Manusmriti, the sixth-century CE Jain scholar and doxographer Haribhadra, provided a different perspective in his writings on Āstika and Nāstika. Haribhadra did not consider "reverence for Vedas" as a marker for an Āstika. He and other first-millennium CE Jaina scholars defined Āstika as one who "affirms there exists another world, transmigration exists, virtue (punya) exists, vice (paapa) exists."

The seventh-century scholars Jayaditya and Vamana, in Kasikavrtti of Pāṇini tradition, were silent on the role of or authority of Vedic literature in defining Āstika and Nāstika. They state, "Āstika is the one who believes there exists another world. The opposite of him is the Nāstika."P. Haag and V. Vergiani (Eds., 2009), Studies in the Kāśikāvṛtti, Firenze: Società Editrice Fiorentina,