The idea expressed in Ian McEwan's "Atonement" is that beauty is limited in its forms and manifestations. The protagonist realizes that while beauty can be categorized into a small range, ugliness is far more complex and diverse, presenting countless variations. This observation reflects a deeper philosophical thought about aesthetics and human perception.

This contrast between beauty and ugliness suggests that our understanding of these concepts is often simplistic. Ugliness, being multifaceted, represents a wider spectrum of experiences and interpretations, while beauty is confined to a narrow definition. This insight invites readers to reflect on how they perceive the world around them, questioning the societal constructs of beauty and the richness found in imperfection.