Verse 4.10.5

स होवाच विजानाम्यहं यत्प्राणो ब्रह्म कं च तु खं च न विजानामीति ते होचुर्यद्वाव कं तदेव खं यदेव खं तदेव कमिति प्राणं च हास्मै तदाकाशं चोचुः ॥ ४.१०.५ ॥
॥ इति दशमः खण्डः ॥

sa hovāca vijānāmyahaṃ yatprāṇo brahma kaṃ ca tu khaṃ ca na vijānāmīti te hocuryadvāva kaṃ tadeva khaṃ yadeva khaṃ tadeva kamiti prāṇaṃ ca hāsmai tadākāśaṃ cocuḥ || 4.10.5 ||
|| iti daśamaḥ khaṇḍaḥ ||

5. Upakosala said: ‘I know that prāṇa is Brahman. But that ka and kha are Brahman I do not know.’ The fires replied, ‘That which is ka is also kha, and that which is kha is also ka.’ Then the fires taught him that Brahman was both prāṇa and ākāśa [space].

Word-for-word explanation:

Saḥ ha uvāca, he [Upakosala] said; vijānāmi aham yat prāṇa brahma, I know that prāṇa is Brahman; kam ca tu kham ca na vijānāmi iti, but I don’t know that ‘ka’ and ‘kha’ are Brahman; te ha ucaḥ, they [the fires] said; yat vāva kam tat eva kham, that which is ‘ka’ is also ‘kha’; yat eva kham tat eva kam iti, that which is ‘kha’ is also ‘ka’; prāṇam ca ha asmai tat ākāśam ca ucaḥ, then they taught him about prāṇa and ākāśa [space]. Iti daśamaḥ khaṇḍaḥ, here ends the tenth section.

Commentary:

Upakosala had no difficulty accepting that prāṇa was Brahman, for prāṇa is the vital breath and Without the vital breath, life is impossible. In view of its importance it may be conceded that prāṇa is Brahman. But how can ka and kha be Brahman?

Upakosala thought to himself: ‘Ka is happiness, but what kind of happiness? It is happiness born of sense experience. It is therefore transitory. It cannot be the same as Brahman. Similarly, kha is also transitory, for it means ākāśa, space. Ākāśa is material and therefore transitory.’

The fires then said that ka and kha are used as both nouns and adjectives, and sometimes they qualify each other. Kha as an adjective may qualify ka when ka stands for Brahman. What does ka mean here? Here it means Brahman without any attributes. Similarly, when we say kha, we mean ākāśa. When we say ka is kha, we mean ‘pleasure’ is ākāśa. But this is not the material ākāśa. Here it means the ākāśa, space, inside the heart. The idea is that Brahman and joy (ka) are both in the space inside the heart (khā).