Upanishads
A corpus of late Vedic Sanskrit texts of composite and largely anonymous authorship, composed over several centuries — the oldest core probably between the eighth and sixth centuries BC, with later layers extending well into the common era — regarded as the philosophical and mystical culmination of Vedic revelation (hence also called Vedanta, 'end of the Vedas'), and constituting the textual foundation of virtually all subsequent schools of Indian philosophy, both orthodox and heterodox. The Sanskrit term 'upanishad', literally meaning 'sitting near' (referring to the student sitting at the teacher's feet to receive secret teachings), reflects the original esoteric, initiatory nature of these texts, composed as philosophical dialogues between teachers and disciples, or between rival sages in debates at royal courts, which progressively abandon the earlier Vedas' concern with external sacrificial ritual to focus on radical metaphysical and existential questions: what is the ultimate nature of reality, what is the self (Atman), and how does it relate to the ultimate ground of the universe (Brahman). Their central philosophical contribution, developed through memorable poetic formulations such as 'tat tvam asi' ('thou art that') from the Chandogya Upanishad, is the essential identification of the individual Atman with the universal Brahman, a doctrine forming the core of what Adi Shankara would later systematise as Advaita Vedanta (non-dualism). The Upanishads also introduce or decisively develop concepts fundamental to all subsequent Indian religious tradition, such as karma (the law of moral cause and effect determining rebirth), samsara (the cycle of death and rebirth) and moksha (final liberation from this cycle through knowledge of the identity between Atman and Brahman). Although more than two hundred versions exist, a group of ten to thirteen 'principal Upanishads' — including the Brihadaranyaka, the Chandogya, the Katha and the Isha — is regarded as the oldest and philosophically most significant canonical core, and their influence extends far beyond Hinduism, having inspired Western thinkers such as Schopenhauer, who considered them 'the solace of my life' and would be 'the solace of my death'.