Ganaka Vaideha said: 'When the sun has set, O Yagnavalkya, and the moon has set, and the fire is gone out, and the sound hushed, what is then the light of man?' Yagnavalkya said: 'The Self indeed is his light; for, having the Self alone as his light, man sits, moves about, does his work, and returns.'

In plain words: The king asks what light is left when the sun, moon, fire and even sound are all gone. Yajnavalkya answers that the Self, a person's own inner being, is the light by which they live and act.