Taittirīya Upaniṣad — The Scripture of Infinite Bliss
The Taittirīya Upaniṣad (Sanskrit: तैत्तिरीय उपनिषद्) belongs to the Taittirīya branch of the Kṛṣṇa Yajurveda — the 'taittirīya' school, traditionally named after the partridge (tittiri). It is universally recognized as one of the ten mukhya (principal) Upaniṣads — the set that Śaṅkara commented upon and that Rāmānuja's Śrī Bhāṣyam repeatedly engages as authoritative revelation. The text is divided into three vallīs (sections): Śikṣā Vallī (instruction in phonetics, conduct, and the duties of a student upon completion of his studies — including the celebrated Convocation Address that is still recited at Indian graduations); Brahmānanda Vallī (also called Ānanda Vallī), which contains the pañcakoṣa doctrine and the ānandamīmāṃsā; and Bhṛgu Vallī, which narrates the progressive realization of Brahman by Bhṛgu, son of Varuṇa, through successive meditations guided by his father.
The Brahmānanda Vallī contains several of the most frequently cited passages in Viśiṣṭādvaita theology. Its opening declaration — satyaṃ jñānam anantaṃ brahma ('Brahman is truth, knowledge, infinite') — becomes a foundational formula for Rāmānuja's understanding of Brahman's nature: not abstract impersonal Absolute, but the omniscient, truth-constituted personal Being who is infinite. The Vallī's pañcakoṣa teaching — the five 'sheaths' (anna, prāṇa, mana, vijñāna, ānanda) progressively nested within each other — is interpreted by Rāmānuja not as layers of illusion to be peeled away (the Advaita reading) but as an ordered disclosure of increasing subtlety, culminating in the ānandamaya koṣa which points beyond itself to Brahman as the innermost indweller. The declaration raso vai saḥ ('He is verily rasa — essence/bliss') is one of the great Upaniṣadic affirmations of Bhagavān's essential nature.
The ānandamīmāṃsā of the Brahmānanda Vallī is the most philosophically elaborate section: it constructs a systematic hierarchy of ānanda beginning with the ānanda of a young, healthy, learned, prosperous human and multiplying it by 100 at each successive level — human gandharvas, divine gandharvas, pitṛs, ājānajas (born-divine), devas-by-action, devas-by-birth, Indra, Bṛhaspati, Prajāpati, and finally Brahman. The conclusion: Brahman's ānanda is not quantitatively greater but qualitatively infinite and incommensurable — the source and ground of all lesser ānandas. Rāmānuja's Śrī Bhāṣyam uses this passage to establish that liberation (mokṣa) is an experience of sharing in the infinite ānanda of the divine Person — not identity with Brahman but the liberated soul enjoying Bhagavān's boundless bliss while remaining eternally the Śeṣa (servant) to Bhagavān's Śeṣī (master).