Good design involves High Cohesion, Low Coupling and Conceptual Integrity.
Michelangelo divided his mural into panels: separate, free-standing areas, each of which tells a story.Andrew Hunt and David Thomas, The Art in Computer Programming, 2001
Each part (class, function, service, tool etc.) of a system should do one thing, and do it well.
Unrelated things should remain self-contained.
Together, all parts of the system tell the same story.