Which C.S. Lewis book argues that friendship is one of four kinds of love?

The Four Loves argues that friendship is one of four kinds of love. C.S. Lewis published the book in 1960 after developing ideas first presented in a BBC radio talk series.

Lewis examines four Greek terms or concepts: storge, affection; philia, friendship; eros, romantic or sexual love; and agape, charity or self-giving love. He does not treat these categories as completely isolated, since human relationships can involve more than one kind of love.

The book is often confused with Lewis’s earlier Christian apologetic works because it discusses religious ideas, but its central organization is the analysis of human love. Lewis also warns that natural loves can become distorted when they are treated as ultimate. The Four Loves is therefore both a literary discussion and a moral reflection.