TAITTIRĪYA UPANISHAD · 3.4

मनो ब्रह्मेति व्यजानात् । मनसो ह्येव खल्विमानि
भूतानि जायन्ते । मनसा जातानि जीवन्ति ।
मनः प्रयन्त्यभिसंविशन्तीति । तद्विज्ञाय ।
पुनरेव वरुणं पितरमुपससार ।
अधीहि भगवो ब्रह्मेति । तंहोवाच ।
तपसा ब्रह्म विजिज्ञासस्व । तपो ब्रह्मेति ।
स तपोऽतप्यत । स तपस्तप्त्वा ।

mano brahmeti vyajānāt । manaso hyeva khalvimāni
bhūtāni jāyante । manasā jātāni jīvanti ।
manaḥ prayantyabhisaṃviśantīti । tadvijñāya ।
punareva varuṇaṃ pitaramupasasāra ।
adhīhi bhagavo brahmeti । taṃhovāca ।
tapasā brahma vijijñāsasva । tapo brahmeti ।
sa tapo’tapyata । sa tapastaptvā ।

He knew mind for the Eternal. For from mind alone, it appeareth, are these creatures born and being born they live by mind, and to mind they go hence and return. And when he had known this, he came again to Varouna his father and said “Lord, teach me the Eternal.” But his father said to him “By askesis do thou seek to know the Eternal, for concentration of force is the Eternal.” He concentrated himself in thought and by the energy of his brooding

Sri Aurobindo · CWSA 18, pp. 226, 227