4-3 Yaagyavalkya and King Janak
Once Janak, the King of Videh, had a conversation about the Agnihotra sacrifice and Yaagyavalkya bestowed him a boon to ask a question from him, so --

King Janak asked - "Yaagyavalkya, What provides light to a person?"
"The Sun, Your Majesty." replied Yaagyavalkya
"But when the Sun has set, what then served as the source of light?"
"The Moon."
"Quite so." Janak commended. "But when the Sun and the Moon both have set what served as the source of light to a person?"
"The fire."
"What happens when the Sun and the Moon have set, and the fire has also gone out?"
"Then the sound of speech serves as the light."
"But when all these are not available what then serves as the source of light for a person?"
"The man's soul." replied Yaagyavalkya.

4-4-5
"The individual soul is, indeed, (part of) the Supreme Soul (Brahm). The individual soul does get attached to (the body and hence with) the faculties of understanding, light, hearing, desire, anger, calmness, right conduct and wrong conduct and with the perception of Earth, Water, Ether, Air, Light etc. This is what is meant when it is said that the individual soul is attached to this and to that. Accordingly, as one does, so one does become. One becomes become virtuous by virtuous actions and vicious by evil actions. A person consists of desires which direct his actions. If we do not break away from sinful actions we cannot know God. He is indeed, paid according to what he earns.