The ritual scholars of Mīmāṃsā, the school that read the Vedas as a body of commands, held that scripture exists to tell people what to do: which sacrifice to perform, when, and for what reward. On their reading even the Upanishads, the philosophical texts that close the Vedas, mattered only as far as they served some act. Śaṅkara, the most famous teacher of Advaita Vedānta, the non-dualist school of Vedānta, built his work on denying that premise. The Upanishads, he argued, command nothing. They describe something already real: the self, which is Brahman, the one ultimate reality, pure awareness without qualities. Because the self already is what it is, no ritual and no good deed can produce it. What frees a person is knowledge, and that knowledge does only one thing: it removes a mistake.
He names the mistake in the preface to his commentary on the Brahma Sūtras. It is adhyāsa, superimposition: everyone confuses the self, which only witnesses, with the body and mind it witnesses, as a person takes mother-of-pearl for silver. From that confusion come "I" and "mine", and with them the whole world of separate things, which belongs to ignorance, avidyā, rather than to what is finally real. When the Chāndogya Upanishad tells a student "that you are", Śaṅkara takes the sentence as a plain identity: the awareness at the core of the student is the reality behind everything. Reasoning alone cannot establish this: non-duality is known from the Upanishads, which count for him as a source of knowledge in their own right. Argument has a narrower job, clearing away objections, and the commentary on the Brahma Sūtras does it against the Sāṃkhya school, which derives the world from unconscious matter, the Buddhists and the Jains in turn.
Of his life, only the writing is record; the rest comes from the biographies called the Śaṅkaradigvijaya, "Śaṅkara's conquest of the quarters", written between the fourteenth and seventeenth centuries. The tradition they hand down places his birth in a Brahmin family at Kalady, in Kerala, and tells that as a boy of eight, seized by a crocodile while bathing, he won his mother's leave to become a renouncer. It names his teacher as Govinda, a pupil of Gauḍapāda, the earlier Advaita teacher on whose verses about the Māṇḍūkya Upanishad Śaṅkara commented. It has him cross India defeating rivals in debate, found four monasteries and die at thirty-two at Kedarnath in the Himalayas; other accounts place his death at Kanchi or in Kashmir. Many older reference works still print the traditional dates, 788 to 820. Historians now place him in the first half of the eighth century, at about 700 to 750, and admit that no evidence fixes his life with certainty.
Of the more than three hundred works handed down under his name, modern scholars accept only a few. Most are commentaries of the kind called bhāṣya, which explain a root text line by line: the Brahmasūtra-bhāṣya, his principal work; the Bhagavadgītā-bhāṣya; and commentaries on the Bṛhadāraṇyaka, Taittirīya, Aitareya, Kaṭha, Praśna, Īśa and Māṇḍūkya Upanishads, the last together with Gauḍapāda's verses on it. One independent treatise, the Upadeśasāhasrī, a teacher's handbook, is nearly always accepted as his. Tradition also ascribes to him the Vivekacūḍāmaṇi, a poem, but most modern scholars doubt that he wrote it. In the Gītā commentary the renouncer's life becomes doctrine: Krishna tells Arjuna to fight because Arjuna does not yet know the self, and whoever does know it gives up action altogether.
In his own time he was not the best-known Advaita author. Until the tenth century that was Maṇḍana Miśra, an older contemporary trained in Mīmāṃsā. Two of Śaṅkara's pupils, Padmapāda and Sureśvara, wrote commentaries on his work, and later Advaita split into two schools, each grown from a line of commentary on his Brahmasūtra-bhāṣya. Opponents answered him for centuries, and some called him a Buddhist in disguise. Rāmānuja, the founder of qualified non-dualism some centuries later, argued that a doctrine calling all knowledge infected by error undoes itself; Madhva, the thirteenth-century founder of the dualist school, argued that God, souls and the world are really and permanently different. Both wrote their own commentaries on the Brahma Sūtras against his. His fame grew from the fourteenth century, when the kings of Vijayanagara supported the monastery at Sringeri that claims him as founder. In the nineteenth and twentieth centuries Hindu reformers and Western scholars presented Advaita as the thread uniting Hinduism, although most Hindus do not follow it. The heads of several Indian monasteries still bear the title Śaṅkarācārya, "teacher Śaṅkara".