"There are the two, drinking their reward in the world of their own works, entered into the cave (of the heart), dwelling on the highest summit (the ether in the heart). Those who know Brahman call them shade and light; likewise, those householders who perform the Trinâkiketa sacrifice.

In plain words: Two selves live in the cave of the heart: the individual self, who tastes the results of its own actions, and the highest Self. Those who know Brahman (the ultimate reality), and people who do the special Nachiketa fire ritual three times, call them shadow and light.