Adi Shankaracharya's commentaries on the Upanishads, the Brahma Sutras, and the Bhagavad Gita — the texts that gave Advaita Vedanta its systematic philosophical form.
८वीं शताब्दी · 8th century CE
Rather than composing original scripture, Shankaracharya wrote bhashya, line-by-line commentary, arguing through each of the ten principal Upanishads, the Brahma Sutras, and the Gita that Brahman (ultimate reality) and Atman (the individual self) are non-different, the central claim of Advaita, non-dual, Vedanta.
His method of close textual commentary set the template later philosophers like Ramanuja and Madhva would follow even while arguing against his conclusions, making the Shankara Bhashya as much a starting point for rival schools as for his own. Shankaracharya himself is covered separately in this site's Rishi Parampara and Tirtha & Devalaya chapters.