The Upanishads, Part 2 (SBE15), by Max Müller, [1879], at sacred-texts.com

FOURTH ANUVÂKA.

He perceived that mind (manas) is Brahman, for from mind these beings are born; by mind, when born, they live; into mind 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--