JANAKA: Yajnavalkya, what is the light of man?
YAJNAVALKYA: The sun is our light, for by that light we sit, work, go out, and come back.
JANAKA: When the sun sets, what is the light of man?
YAJNAVALKYA: The moon is our light, for by that light we sit, work, go out, and come back.
JANAKA: When the sun sets, Yajnavalkya, and the moon sets, what is the light of man?
YAJNAVALKYA: Fire is our light, for by that we sit, work, go out, and come back.
JANAKA: When the sun sets, Yajnavalkya, and the moon sets, and the fire goes out, what is the light of man?
YAJNAVALKYA: Then speech is our light, for by that we sit, work, go out, and come back. Even though we cannot see our own hand in the dark, we can hear what is said and move toward the person speaking.
JANAKA: When the sun sets, Yajnavalkya, and the moon sets, and the fire goes out and no one speaks, what is the light of man?
YAJNAVALKYA: The Self indeed is the light of man, your majesty, for by that we sit, work, go out, and come back.
from the Brihadaranyaka Upanishad, Chapter IV, The States of Consciousness
as translated by Eknath Easwaran