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Prasthanatrayi (Sanskrit: प्रस्थानत्रयी, IAST: Prasthānatrayī), literally, three sources (or axioms), refers to the three canonical texts of Hindu philosophy, especially of the Vedanta schools. It consists of:[1]

The Upanishads, known as Upadesha prasthana (injunctive texts), and the Śruti prasthāna (the starting point or axiom of revelation), especially the Principal Upanishads.
The Brahma Sutras, known as Nyaya prasthana or Yukti prasthana (logical text or axiom of logic)
The Bhagavad Gita, known as Sadhana prasthana (practical text), and the Smriti prasthāna (the starting point or axiom of remembered tradition)

The Upanishads consist of twelve or thirteen major texts, with a total of 108 texts. The Bhagavad Gītā is part of the Mahabhārata. The Brahma Sūtras (also known as the Vedānta Sūtras), systematize the doctrines taught in the Upanishads and the Gītā.

All the founders of the three major schools of Vedanta, viz, Adi Shankara, Ramanujacharya, and Madhva wrote bhāṣyas (commentaries) on these texts.

All scientific knowledge of transcendence must be supported by:

Sri Caitanya-caritamrta - CC Adi-lila

Vedānta-sūtra is known as nyāya-prasthāna, the Upaniṣads are known as śruti-prasthāna, and the Gītā, Mahābhārata and Purāṇas are known as smṛti-prasthāna. All scientific knowledge of transcendence must be supported by śruti, smṛti and a sound logical basis.

CC Adi 7.106, Purport:

According to learned scholars, there are three different sources of knowledge, which are called prasthāna-traya. According to these scholars, Vedānta is one of such sources, for it presents Vedic knowledge on the basis of logic and sound arguments. In the Bhagavad-gītā (13.5) the Lord says, brahma-sūtra-padaiś caiva hetumadbhir viniścitaiḥ: "Understanding of the ultimate goal of life is ascertained in the Brahma-sūtra by legitimate logic and argument concerning cause and effect." Therefore the Vedānta-sūtra is known as nyāya-prasthāna, the Upaniṣads are known as śruti-prasthāna, and the Gītā, Mahābhārata and Purāṇas are known as smṛti-prasthāna. All scientific knowledge of transcendence must be supported by śruti, smṛti and a sound logical basis.

In transcendental realization there are three divisions of knowledge

Other Books by Srila Prabhupada

Teachings of Lord Caitanya

Teachings of Lord Caitanya, Chapter 19:

According to the Skanda and Vāyu Purāṇas, the word sūtra refers to a condensed work which carries meaning and import of immeasurable strength without any mistake or fault. The word vedānta means "the end of Vedic knowledge." In other words, any book which deals with the subject matter indicated by all the Vedas is called vedānta. For example, the Bhagavad-gītā is vedānta because in the Bhagavad-gītā the Lord says that the ultimate goal of all Vedic research is Kṛṣṇa. Thus one should understand that the Bhagavad-gītā and Śrīmad-Bhāgavatam, which aim only at Kṛṣṇa, are vedānta.

In transcendental realization there are three divisions of knowledge, called prasthāna-traya. That department of knowledge which is proved by Vedic instruction (like the Upaniṣads) is called śruti-prasthāna. Authoritative books indicating the ultimate goal and written by liberated souls like Vyāsadeva (for example, the Bhagavad-gītā, Mahābhārata and Purāṇas, especially Śrīmad-Bhāgavatam, the Mahā-Purāṇa) are called smṛti-prasthāna. From the Vedic literature we understand that the Vedas originated from the breathing of Nārāyaṇa. Vyāsadeva, who is an incarnation of the power of Nārāyaṇa, compiled the Vedānta-sūtra (nyāya-prasthāna), but according to Śaṅkara's commentaries, Apāntaratamā Ṛṣi is also sometimes credited with having compiled the aphorisms of the Vedānta-sūtra. According to Lord Caitanya, the conclusions of the verses of the Pañcarātra and the aphorisms of the Vedānta are one and the same. Since the Vedānta-sūtra is compiled by Vyāsadeva, it should be understood to be spoken by Nārāyaṇa Himself. From all the descriptive literature dealing with the Vedānta-sūtra, it appears that there were many other ṛṣis contemporary with Vyāsadeva who also discussed the Vedānta-sūtra. These sages were Ātreya, Āśmarathya, Auḍulomi, Kārṣṇājini, Kāśakṛtsna, Jaimini and Bādarī, while other sages such as Pārāśarī and Karmandī discussed the Vedānta before Vyāsadeva.

In the first two chapters of the Vedānta-sūtra the relationship between the living entities and the Supreme Lord is explained, and in the Third Chapter the discharge of devotional service is explained. The Fourth Chapter deals with the result of discharging devotional service. The natural commentary on the Vedānta-sūtra is Śrīmad-Bhāgavatam. The great ācāryas of the four Vaiṣṇava communities (sampradāyas)—namely, Rāmānujācārya, Madhvācārya, Viṣṇu Svāmī and Nimbārka—have also written commentaries on the Vedānta-sūtra by following the principles of Śrīmad-Bhāgavatam. The followers of these ācāryas, down to the present day, have written many books following the principles of Śrīmad-Bhāgavatam and accepting it as the natural commentary on the Vedānta. Śaṅkara's commentary on the Vedānta-sūtra, known as the Śārīraka-bhāṣya, is very much adored by the impersonalist scholars, but such materialistic commentaries are completely adverse to the transcendental service of the Lord. Consequently Lord Caitanya said that direct commentaries on the Upaniṣads and Vedānta-sūtra are glorious, but that anyone who follows the indirect path of Śaṅkarācārya's Śārīraka-bhāṣya is certainly doomed.