The Upanishads, Part 1 (SBE01), by Max Müller, , at sacred-texts.com
1. Upakosala Kâmalâyana dwelt as a Brahmakârin (religious student) in the house of Satyakâma Gâbâla. He tended his fires for twelve years. But the teacher, though he allowed other pupils (after they had learnt the sacred books) to depart to their own homes, did not allow Upakosala to depart.
2. Then his wife said to him: 'This student, who is quite exhausted (with austerities), has carefully tended your fires. Let not the fires themselves blame you, but teach him.' The teacher, however, went away on a journey without having taught him.
The student from sorrow was not able to eat.
[paragraph continues] Then the wife of the teacher said to him: 'Student, eat! Why do you not eat?' He said: 'There are many desires in this man here, which lose themselves in different directions. I am full of sorrows, and shall take no food.'
4. Thereupon the fires said among themselves: 'This student, who is quite exhausted, has carefully tended us. Well, let us teach him.' They said to him:
5. 'Breath is Brahman, Ka (pleasure) is Brahman, Kha (ether) is Brahman.'
He said: 'I understand that breath is Brahman, but I do not understand Ka or Kha 1.'
They said: 'What is Ka is Kha, what is Kha is Ka 2.' They therefore taught him Brahman as breath, and as the ether (in the heart) 3.
64:3 The Upakosala-vidyâ teaches first Brahman as the cause, and then in its various forms, and is therefore called âtmavidyâ and agnividyâ.
65:1 I do not understand, he means, how Ka, which means pleasure, and is non-eternal, and how Kha, which means ether, and is not intelligent, can be Brahman.
65:2 The commentator explains as follows:--Ka is pleasure, and Kha is ether, but these two words are to determine each other mutually, and thus to form one idea. Ka therefore does not mean ordinary pleasures, but pleasures such as belong to Kha, the ether. And Kha does not signify the ordinary outward ether, but the ether in the heart, which alone is capable of pleasure. What is meant by Ka and Kha is therefore the sentient ether in the heart, and that is Brahman, while Prâna, breath, is Brahman, in so far as it is united with the ether in the heart.
65:3 And as its ether, i.e. as the ether in the heart, the Brahman, with which prâna is connected. Comm.