Ganaka Vaideha said: 'Who is that Self?' Yagnavalkya replied: 'He who is within the heart, surrounded by the Pranas (senses), the person of light, consisting of knowledge. He, remaining the same, wanders along the two worlds, as if thinking, as if moving. During sleep (in dream) he transcends this world and all the forms of death (all that falls under the sway of death, all that is perishable).
In plain words: The king asks who this Self is. Yajnavalkya says it is the bright, knowing presence inside the heart, surrounded by the senses. It stays the same while it moves between this world and the dream world, and in dreams it goes beyond everything that dies.