What are the motifs in The City of Ember by Jeanne DuPrau?

The need to believe in something is a recurring idea in the story. There were about fifty adults with approximately one hundred infants chosen to populate Ember when it was initially founded. Those adults were older and pledged never to talk about their lives prior to Ember. The children were raised without knowing anything other than what they were told about Ember. The goal was to ensure that the children would live happily in Ember without clinging to their desire to return to a life they learned about through their adoptive parents.