Nachiketa asked Yama there is this doubt about a man when he is dead some says that he exists others that he does not. Evidently there is some indestructible element in man different from body sense organs and the mind which scriptures call Atman or self and whose knowledge releases one from repeated is revealed to Nachiketa by Yama the Lord of Death.
CD-1 Chapter 1 Valli 1 & 2
Valli 3, Mantra 1-6
CD-2 Chapter 1 Valli 3 Mantra 7-17
Chapter 2 Valli 4 to 6