A dialogue between King Janaka and his Guru, Yagnavalkya:
Janaka: Yagnavalkya, what serves as the light for man?
Yagnavalkya: The light of the sun, Your Majesty; for by the light of the sun man sits, goes out, does his work and returns home.
Janaka: True indeed, Yagnavalkya. But when the sun has set, what serves then as his light?
Yagnavalkya: The moon is then his light.
Janaka: When the sun has set, O Yagnavalkya, and the moon has set, what serves then as his light?
Yagnavalkya: The fire is then his light.
Their dialogue regresses through a series of lights, resolving itself in the ultimate and irreducible light, the light of the Self. That Self is the Conscious Heart of the universe.