In "Cities of the Plain," Cormac McCarthy explores the theme of memory and the passage of time through the interactions between a child and an adult. The adult grapples with the challenge of responding to a child’s innocent questions about a world that seems distant or nonexistent to him. This reveals a deep reflection on the nature of memory and existence, suggesting that one's past can fade to the point of becoming irrelevant or forgotten.

The adult’s struggle to answer the child's inquiries highlights the complexity of understanding one's own life experiences. It underscores the idea that the past may not always remain accessible, influencing how individuals connect with the present and the future. This interaction serves as a poignant reminder of the transient nature of life and the enduring questions that arise from our existence.