Taittirīya Upaniṣad · 4
यश्छन्दसामृषभो विश्वरूपः । छन्दोभ्योऽध्यमृतात् संबभूव । स मेन्द्रो मेधया स्पृणोतु । अमृतस्य देव धारणो भूयासम्
yaśchandasāmṛṣabho viśvarūpaḥ | chandobhyo'dhyamṛtāt saṃbabhūva | sa mendro medhayā spṛṇotu | amṛtasya deva dhāraṇo bhūyāsam
Who, of all forms, the bull of the chants, emerged from the immortal chants — may He, the Lord, inspire me with intelligence. Possessor of the immortal, oh God, may I be.
This anuvāka establishes the power of the Praṇava, the sacred syllable Om, as the means to acquire medhā (retentive intelligence) — the capacity to remember and comprehend spiritual teachings.
Om is the “bull of the chants” because it dominates among all mantras, as the bull dominates the herd. It is viśvarūpa (of all forms) because it contains all the diversity of the universe within itself. It emerged from the “immortal” Vedas and is, in turn, their essence.
For the yoga practitioner, this indicates that the recitation of Om purifies the mind and develops the retentive capacity necessary for deep contemplation. The medhā here is not mere intellectual memory, but the intuitive capacity to understand the truth of the Ātman.
The desire to be “possessor of the immortal” points to the ultimate goal: mokṣa, liberation from death through the knowledge of Brahman.