Three foundational texts
The prasthāna-traya — triple foundation — of Vedānta is the Upaniṣads (the philosophical conclusions of the Vedic corpus), the Brahma Sūtras (a terse second-century BCE summary of those conclusions, attributed to Bādarāyaṇa), and the Bhagavad Gītā (the practical-spiritual dialogue from the Mahābhārata). Every major Vedāntic school produces a commentary on each, and the commentaries are how the schools' positions are precisely identified.
The three sub-schools
Advaita (non-dual) Vedānta, systematised by Ādi Śaṅkara in the eighth century, treats the individual soul (ātman) and the absolute (brahman) as identically the same — that thou art (tat tvam asi). Viśiṣṭādvaita (qualified non-dual) Vedānta, Rāmānuja in the eleventh-twelfth centuries, treats them as inseparable but distinguishable — like body and self. Dvaita (dual) Vedānta, Madhva in the thirteenth century, treats them as eternally distinct. The disagreements are technical and have produced literature in the millions of words.
The Western inheritance
Almost the entire English-language non-dual tradition that runs through Ramana Maharshi, Nisargadatta Maharaj, Jean Klein, Francis Lucille and Rupert Spira is some local refraction of advaita Vedānta. The vocabulary (ātman, brahman, Self, awareness) and the structural moves (the inquiry into the experiencer, the recognition of unity) are advaitic. Knowing the parent tradition makes the inheritances much easier to read.
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