Hindu philosophy

Hindu philosophy or Vedic philosophy is the set of philosophical systems that developed in tandem with the first Hindu religious traditions during the iron and classical ages of India. In Indian philosophy, of which Hindu philosophy is a prominent subset, the word used for philosophy is Darshana, from the Sanskrit root 'दृश' meaning 'to see, to experience'.
The schools of thought or Darshanas within Hindu philosophy largely equate to the six ancient orthodox schools: the āstika schools, defined by their acceptance of the Vedas, the oldest collection of Sanskrit texts, as an authoritative source of knowledge. Of these six, Samkhya is the earliest school of dualism; Yoga combines the metaphysics of Samkhya with meditation and breath techniques; Nyaya is a school of logic emphasising direct realism; Vaisheshika is an offshoot of Nyaya concerned with atomism and naturalism; Mimamsa is a school justifying ritual, faith, and religious obligations; and Vedanta contains various traditions that mostly embrace nondualism.
The Indian philosophical landscape during the ancient and medieval periods also produced philosophical systems that share many concepts with the āstika traditions, yet at the same time reject or oppose several central Vedic concepts, such as Ātman, or interpret them in their own way. These have been called nāstika philosophies, and they include: Buddhism, Jainism, Charvaka, Ajivika, and others. These nāstika schools of thought are thus regarded as Indian but almost never as Hindu. Western scholars have debated the relationship and differences within āstika philosophies and with the nāstika philosophies, starting with the writings of Indologists and Orientalists of the 18th and 19th centuries, based on limited availability of Indian literature and medieval doxographies. The various sibling traditions included in Indian philosophies are diverse and are united by: shared history and concepts, textual resources, ontological and soteriological focus, and cosmology. Some ambiguity arises from the word Hindu being an exonym historically used as a geographical and cultural identifier for people living in the Indian subcontinent, sometimes regardless of their beliefs.
Hindu philosophy also includes several sub-schools of theistic philosophies that integrate ideas from two or more of the six orthodox philosophies. Examples of such schools include: Pāśupata Śaiva, Śaiva siddhānta, Pratyabhijña, Raseśvara and Vaiṣṇava. Some sub-schools share Tantric ideas with those found in some Buddhist traditions, which are nevertheless found in the Puranas and the Āgamas. Each school of Hindu philosophy has extensive epistemological literature called Pramana, as well as theories on metaphysics, axiology, and other topics.

Classifications

In the history of India, the six orthodox schools had emerged before the start of the Common Era, and some schools emerged possibly even before the Buddha. Some scholars have questioned whether the orthodox and heterodox schools classification is sufficient or accurate, given the diversity and evolution of views within each major school of Indian philosophy, with some sub-schools combining heterodox and orthodox views.
Since ancient times, Indian philosophy has been categorised into āstika and nāstika schools of thought. The orthodox schools of Indian philosophy have been called ṣaḍdarśana. This schema was created between the 12th and 16th centuries by Vedantins. It was then adopted by the early Western Indologists, and pervades modern understandings of Indian philosophy. Satoshi Ogura thus warns that we should keep in mind the tendency of classification of Indic knowledge in Persianate discourses and its legacies in modern writings in both India and the Western world.

Āstika

There are six āstika schools of thought. Each is called a darśana, and each darśana accepts the Vedas as authority. Each āstika darśana also accepts the premise that Atman exists. The schools of philosophy are:

Samkhya – A strongly dualist theoretical exposition of consciousness and matter. Agnostic with respect to God or the gods.
Yoga – A monotheistic school which emerged from Samkhya and emphasises practical use of Samkhya theory: meditation, contemplation and liberation.
Nyaya or logic – The school of epistemology which explores sources of knowledge.
Vaisheshika – An empiricist school of atomism.
Mīmāṃsā – An anti-ascetic and anti-mysticist school of orthopraxy. This tradition is also known as Pūrva-Mīmāṁsā because of its focus on the earlier Vedic texts dealing with ritual actions, and similarly also known as Karma-Mīmāṁsā due to its focus on ritual action.
Vedanta – They focus on the last segment of knowledge in the Vedas, or jñānakāṇḍa. Vedanta is also referred to as Uttara-Mimamsa. Vedānta came to be the dominant current of Hinduism in the post-medieval period. This school considers the Upanishads, the Brahma Sutras, and the Bhagvad Gita as authoritative texts.
Nāstika

Schools that do not accept the authority of the Vedas are nāstika philosophies, of which four schools are prominent:

Charvaka, a materialism school that accepted the existence of free will.
Ājīvika, a materialism school that denied the existence of free will.
Buddhism, a philosophy that denies existence of ātman and is based on the teachings and enlightenment of Gautama Buddha.
Jainism, a philosophy that accepts the existence of the ātman, and is based on the teachings and enlightenment of twenty-four teachers known as tirthankaras, with Rishabha as the first and Mahavira as the twenty-fourth.
Other schools

Besides the major orthodox and non-orthodox schools, there have existed syncretic sub-schools that have combined ideas and introduced new ones of their own. The medieval scholar Madhavacharya, identified by some as Vidyaranya, in his book 'Sarva-Darsana-Sangraha', includes 16 philosophical systems current as of 14th century. Along with some of the major orthodox and non-orthodox schools and sub-schools, it includes the following sub-schools:

Pashupata Shaivism, developed by Nakulisa
Shaiva Siddhanta, the theistic Sankhya school
Pratyabhijña, the recognitive school of Kashmir Shaivism, Trika
Raseśvara, a Shaiva school that advocated the use of mercury to reach immortality
The Pāṇinīya

The above sub-schools introduced their own ideas while adopting concepts from orthodox schools of Hindu philosophy such as realism of the Nyāya, naturalism of Vaiśeṣika, monism and knowledge of Self as essential to liberation of Advaita, self-discipline of Yoga, asceticism and elements of theistic ideas. Some sub-schools share Tantric ideas with those found in some Buddhist traditions.