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We now feature three key Sacred Texts, which are said to constitute the Prasthana Traya (Prasthānatrayī), the “three sources” or “three institutes” of Vedanta according to the Advaita Vedanta school of Indian Philosophy.
The Prasthānatrayī consist of 1) the Principle (Mukhya) Upanishads, 2) the Bhagavad Gita, and 3) the Brahma Sutra. A description of their traditional origin as the three sources is well explained by T. S. Narayana Sastry:
Of all these schools [Darśanas] of philosophy, the most important and authoritative was the Vedanta Darśana founded by Bādarāyaṇa, son of Bādari, at Badarikāṡrama on the Himalayas. He found that the existing schools of philosophy—both orthodox and unorthodox—were in some way or other opposed to the true spirit of the Śrutis and Śmṛitis as handed down by Veda Vyāsa or Kṛishṇa Dvaipāyana, the compiler of the Vedas and the author of the Mahabharata. He at once understood that the true ideal of the Śrutis lay in the unity of the Infinite in man and the Infinite in nature and that this was fully declared and established in the Āraṅyaka or Upanishad portion of the Vedas and not in the Samhitā and Brabmaṇa portions of the same.
Like the great Veda Vyāsa, Bādarāyaṇa divided the whole of the Vedas into two main parts—the Karmakāṇda and the Jñānakāṇda—the exoteric and the esoteric, and arranged the latter into Ten Principal Upanishads. Again he separated the Bhagavadgītā from the rest of the Mahābhārata and made it—perhaps with slight alterations—an independent authority like the Upanishads for his new school of philosophy. But bare quotations from these Śrutis and Śmṛitis were not enough to satisfy the growing demands of the intelligent inquirers of his time. Further, there were apparent inconsistencies between the various texts of the Upanishads and the Bhagavadgītā, which required to be reconciled and explained away by means of strict reasoning. To reduce, therefore, the teachings of the Upanishads and of the Bhagavadgītā or the Mahābhārata to a consistent and systematic whole, to explain away apparent contradictions in those various texts, and to refute all objections that have been or might be urged against them, it was necessary for Bādarāyaṇa that he should compose a work strictly based on reasoning. He accordingly composed his famous Vedānta Sūtras in four chapters in 556 aphorisms. Each of these four chapters (Adhyāyas) comprises in its turn four sections (Padas) and each section a number of sub-sections (Adhikaraṇas).
These three—the Upanishads, the Bhagavadgītā and the Vedānta Sūtras—constitute, according to Bādarāyaṇa, the complete canon of the Vedānta Darśana. They are called the three Prasthānas or Institutes of Vedāntic teaching, the Upanishads being called the Śruti-Prasthāna (Scriptural Institute); the Bhagavadgītā, the Smṛiti-Prasthāna (Traditional Institute); and the Vedānta Sūtras, the Nyāya-Prasthāna (Logical Institute).
—from The Age of Sankara, by T. S. Narayana Sastry, p. 42-47
We’ve put together a page for each “source”, composed of multiple translations and commentaries, and invite the student to make a study of these works, which are sure to shed much light on the path.