At the same time as she takes the philosophy course, Sophie receives a strange postcard sent to Hilde Møller Knag, care of Sophie. The postcard is from Hilde's father and wishes Hilde happy birthday. Sophie is confused, and moreso when she finds a scarf with Hilde's name on it. She does not know what is happening but she is sure that Hilde and the philosophy course must somehow be connected. She learns about Socrates, who was wise enough to know that he knew nothing. Then Alberto sends her a video that shows him in present day Athens and somehow he seems to go back in time to ancient Athens. She learns about Plato and his world of ideas and then about Aristotle, who critiqued Plato, classified much of the natural world, and founded logic and our theory of concepts.
Then, as Sophie's education continues, the Hilde situation begins to get more complicated. She finds many more postcards to Hilde, and some of them are even dated on June 15, the day of Sophie will turn 15. The problem is that June 15 is still over a month away. She discovers some of this with her best friend Joanna, and one of the postcards tells Hilde that one day she will meet Sophie and also mentions Joanna. Strange things are happening that the girls cannot figure out. Sophie's relationship with her mother becomes somewhat strained as she tries both to cover up the correspondence with Alberto and to practice her philosophical thinking on her mom. Meanwhile, Alberto teaches Sophie about Jesus and the meeting of Indo-European and Semitic culture. She learns about St. Augustine, St. Aquinas, and the christianization of Greek philosophy that occurred in the Middle Ages. By this time, Sophie has met Alberto and he begins hinting that the philosophy is about to get extremely relevant to the strange things that are happening to her.
Sophie learns about the focus on humanity in the Renaissance and the extremes of the Baroque and then Alberto focuses on some key philosophers. Urgently, he teaches her about Descartes, who doubted, and by doing so knew at least that he could doubt. They move on to Spinoza as it becomes clear that Hilde's father has some awesome power over them. Then Sophie learns about the empiricists. Locke believed in natural rights and that everything we know is gained from experience. Hume, an important influence on Kant, showed that our actions are guided by feelings and warned against making laws based upon our experiences. But Berkeley is most important to Sophie because he suggested that perhaps our entire lives were inside the mind of God. And Alberto says that their lives are inside the mind of Albert Knag, Hilde's father.
At this point the story switches to Hilde's point of view. On June 15, the day she turns fifteen, Hilde receives a birthday gift from her father entitledSophie's World. She begins to read and is...