In "The Secret Cause", Machado de Assis reveals the dark side of human nature through the love triangle formed by Garcia, Fortunato, and Maria Luísa. While Garcia harbors a silent passion for the young woman, he discovers, to his horror, that Fortunato, her husband, takes secret pleasure in the pain of others. The narrative follows the contrast between genuine love and cold cruelty, culminating in the disturbing revelation of the "secret cause" that drives Fortunato: delight in suffering, which Machado exposes with subtle irony and profound moral unease.