The Ten Major Upanishads — The Prasthana Trayi Foundation
The ten major Upanishads commented upon by Shankara form the Shruti foundation of Advaita Vedanta — the revealed texts whose authority grounds all subsequent philosophical development.
The ten major Upanishads on which Shankara wrote commentaries are: Isha, Kena, Katha, Prashna, Mundaka, Mandukya, Taittiriya, Aitareya, Chandogya, and Brihadaranyaka. Together with the Bhagavad Gita (Smriti) and the Brahmasutras (Nyaya Prasthana), they form the Prasthana Trayi (the three starting points) of Vedantic study. Shankara's commentaries on all three are the foundational texts of Advaita Vedanta. Each Upanishad contributes unique insights to the Advaita vision: the Mandukya is considered sufficient for liberation by itself (Gaudapada); the Chandogya provides the most systematic pedagogical account (Tat Tvam Asi); the Brihadaranyaka the most philosophically rigorous (neti neti); the Katha the most poetically powerful (chariot analogy); the Taittiriya the most systematic on the levels of the Self (Pancha Kosha). Together they constitute one of the most extraordinary collections of spiritual philosophy ever compiled.
The Upanishads are the knowledge of knowledge — by which all knowledge is crowned and given its foundation.
Philosopher Arthur Schopenhauer, who first encountered the Upanishads through Anquetil Duperron's Latin translation, declared: 'In the whole world there is no study so beneficial and so elevating as that of the Upanishads... access to them has been for me the greatest privilege this century has to offer.'
Mundaka Upanishad