Verse 3.17.7-8

आदित्प्रत्नस्य रेतसः ज्योतिः पश्यन्ति वासरम्; परो यदिध्यते दिवि ॥ ३.१७.७ ॥
उद्वयं तमसस्परि ज्योतिः पश्यन्त उत्तरंस्वः पश्यन्त उत्तरं देवं देवत्रा सूर्यमगन्म ज्योतिरुत्तममिति ज्योतिरुत्तममिति ॥ ३.१७.८ ॥
॥ इति सप्तदशः खण्डः ॥

āditpratnasya retasaḥ jyotiḥ paśyanti vāsaram; Paro yadidhyate divi || 3.17.7 ||
udvayaṃ tamasaspari jyotiḥ paśyanta uttaraṃsvaḥ paśyanta uttaraṃ devaṃ devatrā sūryamaganma jyotiruttamamiti jyotiruttamamiti || 3.17.8 ||
|| iti saptadaśaḥ khaṇḍaḥ ||

7-8. [Those who know Brahman] see that the light shining in Para-Brahman is the seed of the world. This light is all-pervasive like daylight. It is eternal. It is that great light which is the cause of the world.

Word-for-word explanation:

Ādit, [has no meaning except that it introduces a new line of thought]; pratnasya, ancient; retasaḥ, seed [of the world]; jyotiḥ, light [manifestation]; paśyanti, they see [eveṛywhere]; vāsaram, daylight [i.e., as all-pervasive]; paraḥ, the best; yat, that which; idhyate, shines; divi, in heaven [i.e., in Para-Brahman], Tamasaḥ, darkness [i.e., ignorance]; pari, beyond [dispels]; uttaram, the best; jyotiḥ, light; paśyantaḥ, having seen; svaḥ, in oneself [i.e., in one’s own heart]; devam, bright; devatrā, among all the gods; sūryam, the sun; vayam ut-aganma, we have attained; jyotiḥ uttamam iti jyotiḥ uttamam iti, the best light, the best light. Iti saptadaśaḥ khaṇḍaḥ, here ends the seventeenth section.

Commentary:

The greatest light is that which dispels the darkness of ignorance. Having seen this light within our own hearts, we have attained that shining god who is the Supreme Light of all.

There is an eternal light which is the cause of this universe. This light is the source of the light we see in the luminous Jodies such as the sun, the moon, and the stars. This light is everywhere. Everything in this phenomenal universe moves and works the way it does because of this light.

Those who have lived a life of self-discipline, with their minds on the highest thoughts, and have thereby acquired a pure heart, can see this light. They see it within themselves and they also see it outside. They know this light as their own Self and as the Self of all. They know this light as Para Brahman.