On Thursday morning, Sophie reads the new letter from Alberto. He explains that he left the postcards to Hilde in the cabin because he thought she would return, and he also refers to June 15th in a way that makes it seem as if it will be a special day. He says they will meet soon. His letter is about Jesus of Nazareth. Alberto starts by explaining that the Greeks and Romans are a part of Indo-European culture, while the Jews belong to Semitic culture. He describes how Indo-European culture (which covers most of Europe) was characterized by a belief in many gods—pantheism. Similar ideas popped up in many different Indo-European languages, and were expressed by words that resembled each other greatly. Sight was the most important of the senses for Indo-European culture. The Semites, on the other hand, are characterized by monotheism, the belief in one god. Judaism, Islam, and Christianity are all Semitic religions. But Christianity complicates things, because it spread throughout Indo-European cultures and incorporated many features of those cultures. Sophie learns the historical context leading up to Jesus—the fact that for almost a thousand years before his birth Jews in Israel prophesied a Messiah. Jesus comes as the Messiah, not only to the people of Israel, but for all mankind. He showed that one could not earn salvation but that God is merciful and will forgive all who ask for forgiveness. Sophie learns about Paul, who converted to Christianity and then spread it to many places, including Athens. Alberto tells Sophie he wants her to be aware of her historical roots and Sophie realizes that such knowledge will greatly enrich her.
After a week with nothing else from Alberto, on Friday May 25th, a postcard from Hilde's father lands on her windowpane. It dates from June 15th, and he tells Hilde he hopes it is still her birthday, and that a "week or two for Sophie does not have to mean just as long for us." He also tells her to say hello to Sophie, who, unfortunately, does not yet understand everything that Hilde perhaps does. Soon after, Sophie gets a call from Alberto, who tells her they must meet in person since Hilde's father is getting too close to them. She sleeps over at Joanna's and then goes to meet him from her house. Although she does not understand him, he says Berkeley will be the key figure and that they must get Hilde on their side before her father returns. The next morning, she meets Alberto at a church where he tells her about the ten centuries of the Middle Ages. Although people in the Renaissance called this time the Dark Ages, Alberto points out that universities and schools were established in the Middle Ages. In addition, nation-states became established, with their major cities. There was a period of cultural and population decline, as feudalism set in and bartering once again became the form of payment. But the Pope was set up as head of the Church, and kings began to become very powerful. Greco-Roman culture split up and then came together again in the Renaissance. St. Augustine was a Christian Platonist who brought Plato's philosophy into Christianity. He went out of his way to unite Greek and Jewish thought. His great book was called the City of God and he suggested that salvation came only through the Church. St. Thomas Aquinas brought Aristotle into the Christian religion and he tried to show that reason and faith do not come into conflict. She also learns that one of the female philosophers at this time was named Hildegard, who had a vision in which she saw Sophia, the female side of God. This scares Sophie, as does the fact that Albert the Great was Aquinas' teacher.
Although Alberto's letter to Sophie about Christianity seems to explain the postcards, in a way it only transfers the uncertainty. Many questions remain: how did Alberto know that Sophie would return? And how did a postcard get there postmarked on the day they went to the major's cabin? Neither of these questions are answered, and the mystery of Hilde seems to grow larger. Alberto makes some vague references to something that Sophie does not understand, but it is clear that he knows more than she does and that he is not happy with the situation. We do not know how Alberto has figured out more, and it seems clear that it is Hilde's father who is somehow in control. Gaarder uses an interesting technique to add to the suspense. At the end of the chapter about the postcards things seem to rapidly be spinning out of control. Yet in the first few sentences of the next chapter Alberto takes the responsibility for the postcards. The effect of Alberto's words is to make us think that perhaps everything is not as strange as it appears. But then, a week later, during Sophie's next encounter with Alberto, he reveals that he knows more and that they will inevitably clash with Hilde's father. Furthermore, that encounter will focus on the philosophy of Berkeley, someone Sophie (and therefore the reader) does not even know about. By periodically making it appear as if some of the tension in the plot is not really as critical as it is, Gaarder manages to carry the story at a high level of tension for many chapters without making the reader feel that things have been drawn out too long.
Also, the fact that we know that Alberto is teaching Sophie philosophy in chronological order combined with his certainty that Berkeley is a key figure is a way to make the teaching of the philosophy even more central. We know that Sophie must learn at least all of the way up through Berkeley. But since Sophie learns the philosophy in a manner that makes it accessible to all of us, this ensures that the reader will learn the philosophy and believe it critical to do so. Gaarder has therefore come up with a brilliant solution to the key problem that a novel like Sophie's World must face—if it is to be a novel about the history of philosophy then some way must be found to work the philosophy directly into the plot. For the first several chapters it is enough that the philosophy is transmitted to Sophie through mysterious means and by a mysterious person. However, since there is still much more philosophy to come, it is necessary to link the philosophy lessons inextricably to the plot. And the fact that somehow everything that is occurring within Sophie's world may hinge upon the philosopher Berkeley provides that link. We know that Sophie cannot skip ahead to Berkeley because she must get to him with knowledge of the historical and philosophical context behind him. Suddenly it is not only interesting that Sophie receives strange philosophy lessons—it is urgent that she do so.