At many points in the first part of the book, Lewis uses the symbol of a piano to describe the relationship between humanity and moral law. He argues that the relationship between human instinct and moral law is like the relationship between notes on a piano and sheet music—no instinct is “right” or “wrong” one hundred percent of the time, just as no note on a piano is always right or wrong, either. Notes, like instincts, are guided and controlled by the overarching authority of moral law (sheet music). In all, the piano symbolizes the different levels of human morality: our specific behaviors, our instincts to obey or disobey moral law, and the moral law itself.
The timeline below shows where the symbol The Piano appears in Mere Christianity. The colored dots and icons indicate which themes are associated with that appearance.
Book 1, Chapter 2: Some Objections
Book 4, Chapter 2: The Three-Personal God