Chāndogya Upaniṣad — The Sāmaveda's Jewel of Brahma-Vidyā
Chāndogya Upaniṣad ('the Upaniṣad pertaining to the Chāndogas — the chanters of the Sāmaveda'; from chandas = song, verse, with reference to the Sāmaveda singers) is one of the two longest and most philosophically rich principal Upaniṣads (alongside the Bṛhadāraṇyaka), belonging to the Sāmaveda. It consists of eight prapaṭhakas (chapters) covering a vast range of topics: the philosophy of the sacred syllable Oṃ, the nature of Brahman as ākāśa (space), prāṇa (vital breath), ānanda (bliss), satyam (truth) and vijñānam (consciousness), the doctrine of ṣaḍja and sāman (foundational Sāmavedic musical categories), and many cosmological and meditative teachings. The Chāndogya Upaniṣad contains the complete Gāyatrī meditation, the story of Satyakāma Jābāla, the Pañcāgni Vidyā (doctrine of the five fires), and many other teachings central to the Upaniṣadic tradition.
For Viśiṣṭādvaita, the Chāndogya is of supreme importance through two foundational passages. The first is the celebrated Ṣaḍ-vidyā section (Chāndogya 6), where the sage Uddālaka Āruṇi teaches his son Śvetaketu the nature of Brahman as Sat — pure Being — from which all existence emerges at the beginning of a cosmic cycle and into which all returns at the end. Uddālaka's teaching 'Sadeva somyedam agra āsīd ekam evādvitīyam' ('In the beginning, dear one, this was Being alone — one only, without a second') affirms the ultimate unity of all existence in Brahman. The famous mahāvākya — 'tat tvam asi' ('That thou art') — appears in this section no fewer than nine times, as Uddālaka repeats it to drive home the teaching that the jīva (individual self) is ultimately one with Brahman.
Rāmānuja's interpretation of tat tvam asi stands as one of the most significant contributions of Viśiṣṭādvaita to Indian philosophy. Against Śaṃkara's interpretation that the statement asserts the absolute identity of the individual self and the attributeless (nirguṇa) Brahman (the jīva is numerically identical to Brahman once māyā is removed), Rāmānuja argues — through rigorous grammatical, contextual, and philosophical analysis — that tat refers to Brahman as the inner self (antaryāmin) of all existence, and tvam refers to the jīva as a conscious entity that is Brahman's śeṣa (owned and supported one), śarīra (body), and viśeṣaṇa (attribute/mode). Thus tat tvam asi affirms not absolute identity but viśiṣṭa-ābhedha — the qualified non-difference: the jīva and Brahman are non-different in the sense that the jīva is Brahman's mode of being, inseparably related as body to soul. This interpretation — known in the tradition as sāmānādhikaraṇya (grammatical co-predication) with the meaning 'the jīva is the body of Brahman' — preserves both the reality of the individual self and the supremacy of Bhagavān.