The longest continuously debated philosophical tradition on earth: roughly 2,500 years of argument about knowledge, self, and liberation, carried first in the oral traditions and Vedic corpus of the Vedic age, then in the competing schools of the classical period. Its distinctive move is that epistemology comes first: nearly every school is defined by which pramāṇas — means of knowledge — it accepts.
The ritual hymns of the Vedas turn philosophical in the Upaniṣads, where questions about Brahman and the self displace ritual procedure.
The fifth century BCE sees a burst of heterodox movements — Buddhism, Jainism, and the materialist Cārvāka — that reject Vedic authority outright.
Classical India answers with six orthodox (āstika) systems that accept the Veda: Sāṃkhya, Yoga, Nyāya, Vaiśeṣika, Mīmāṃsā and Vedānta — each with its own root text and commentator lineage.
The tradition stays argumentative into the modern era: Mādhava's Sarva-darśana-saṃgraha surveys sixteen schools as rivals, not one voice.
Central Ideas
Pramāṇa theory: what counts as a source of knowledge — perception, inference, testimony — is the spine of the debate; the Nyāya school built a five-membered inference schema studied alongside Greek logic.
Liberation: nearly every school treats ignorance of the true nature of self and world as the problem, and philosophical knowledge as the route out of rebirth — see karma and nirvana.
Dualism and its rivals:Sāṃkhya's split between consciousness (puruṣa) and nature (prakṛti) is answered by Vedānta's non-dualism and by Buddhist emptiness.
Dharma: conduct, duty and cosmic order — the concept of dharma frames ethics as fitting response rather than rule-following alone.