Mortimer came back trembling from a long journey; he loved Jenny; she had not answered her letters. When he arrived in London, he rides a horse and picks her up from his country house. He arrives, she was walking in the park; he runs there, his heart racing; he meets her, she extends her hand to him, receives him with trouble: he sees that he is loved. As she walked through the park alleys with her, Jenny's dress embarrassed herself in a thorny acacia bush. Later on, Mortimer was happy, but Jenny was unfaithful. I maintain that Jenny never loved him; he cites as proof of his love the way she received him when she returned from the continent, but he could never give me the slightest detail. Only he flinches visibly as soon as he sees an acacia bush; it is really the only distinct memory he has kept of the happiest moment of his life.

Under the guise of a psychological and sociological analysis of love, he expresses his unfortunate passion for Matilde Viscontini Dembowski. It is in this book that he invents and describes the famous phenomenon of crystallization.

An essay on the feeling of love in which he tries to categorize and analyze the different types of emotions. The most famous idea is that of "crystallization": the lover sees the loved one under the prism of perfection and makes an idealized image of him.