The Upanishads, Part 2 (SBE15): Taittirîyaka-Upanishad: III, 5
The Upanishads, Part 2 (SBE15), by Max Müller, [1879]
He perceived that understanding (vigñâna) was Brahman, for from understanding these beings are born; by understanding, when born, they live; into understanding they enter at their death.
Having perceived this, he went again to his father Varuna, saying--'Sir, teach me Brahman.' He said to him: 'Try to know Brahman by penance, for penance is (the means of knowing) Brahman.'
He performed penance. Having performed penance--