Adi Shankaracharya was an Indian philosopher, theologian and is believed to be avatar of Lord Shiva whose works had a strong impact on the doctrine of Advaita Vedanta. He founded four mathas, which are believed to have helped in the historical development, revival and propagation of Advaita Vedanta.
According to tradition, he travelled across the Indian subcontinent to propagate his philosophy through discourses and debates with other thinkers, from both orthodox Hindu traditions and heterodox non-Hindu-traditions, including Buddhism, defeating his opponents in theological debates. His commentaries on the Prasthanatrayi Vedic canon argue for the unity of Ātman and Nirguna Brahman "brahman without attributes," defending the liberating knowledge of the Self and the Upanishads as an independent means of knowledge against the ritually-oriented Mīmāṃsā school of Hinduism.
Shankara's Advaita shows similarities with Mahayana Buddhism, despite his critiques; and Hindu Vaishnavist opponents have even accused Shankara of being a "crypto-Buddhist," a qualification which is rejected by the Advaita Vedanta tradition, highlighting their respective views on Atman, Anatta and Brahman.