Chandogya Upanishad in the context of "Brihadaranyaka Upanishad"

The Mahāvākyas (sing.: mahāvākyam, महावाक्यम्; plural: mahāvākyāni, महावाक्यानि) are "The Great Sayings" of the Upanishads, with mahā meaning great and vākya, a sentence. The Mahāvākyas are traditionally considered to be four in number, though actually five are prominent in the post-Vedic literature:

1. Tat Tvam Asi (तत् त्वम् असि) – literally translated as "That Thou Art" ("That is you" or "You are that"), appears in Chandogya Upanishad 6.8.7 of the Sama Veda, with tat in Ch.U. 6.8.7 referring to *sat, "the Existent," and contextually understood as "That's how [thus] you are," with tat in Ch.U. 6.12.3 referring to "the very nature of all existence as permeated by [the finest essence]."
2. Ahaṁ Brahmāsmi (अहं ब्रह्मास्मि) - "I am Brahman", or "I am absolute" (Brihadaranyaka Upanishad 1.4.10 of the Yajur Veda)
3. Prajñānaṁ Brahma (प्रज्ञानं ब्रह्म) - "Prajñāna is Brahman", or "Brahman is Prajñāna" (Aitareya Upanishad 3.3 of the Rig Veda)
4. Ayam Ātmā Brahma (अयम् आत्मा ब्रह्म) - "This Self (Atman) is Brahman" (Mandukya Upanishad 1.2 of the Atharva Veda)
5. Sarvaṃ Khalvidaṃ Brahma - "All this indeed is Brahman"(Chāndogya Upaniṣad 3.14.1)

Mahāvākyas are instrumental in Advaita Vedanta, as they are regarded as valid scriptural statements that reveal the self (ātmán), which appears as a separate individual existence (jīva), is, in essence, non-different (not two-ness) from Brahman, which, according to Advaita, is nirguna. In contrast, these statements are less prominent in most other Hindu traditions, which emphasize a qualified or dualistic relationship between the self and Brahman, whom they regard as saguna, often identified with Vishnu, Shiva, Shakti, etc.

Prasthanatrayi (Sanskrit: प्रस्थानत्रयी, IAST: Prasthānatrayī), literally, three sources (or axioms), refers to the three canonical texts of theology having epistemic authority, especially of the Vedanta schools. It consists of:

1. The Upanishads, known as Upadeśa Prasthāna (injunctive texts), and the Śruti Prasthāna (the starting point or axiom of revelation), especially the Principal Upanishads.
2. The Bhagavad Gita, known as Sādhana Prasthāna (practical text), and the Smṛti Prasthāna (the starting point or axiom of remembered tradition)
3. The Brahma Sutras, known as Sūtra Prasthāna (formulative texts) or Nyāya Prasthāna or Yukti Prasthāna (logical text or axiom of logic)

The Upanishads consist of ten, twelve or thirteen major texts, with a total of 108 texts (some scholars list ten as principal – the Mukhya Upanishads, while most consider twelve or thirteen as principal, most important Upanishads). The ten Upanishads are Īśā, Kena, Kaṭha, Praṣna, Muṇḍaka, Māṇḍūkya, Taittirīya, Aitareya, Chāndogya and Bṛhadāraṇyaka.