Alberto continues talking to Sophie, and he describes the life of Descartes. Descartes decided, much like Socrates, that he did not know very much. He doubted the many philosophical works that had been handed through the Middle Ages and he set out to build his own philosophical system. Descartes was the first philosopher in a long time to attempt to bring all knowledge into a coherent philosophy. His concerns were with certain knowledge—that which we can know for sure—and the mind/body relationship. Because philosophers believed in a mechanistic view of nature, it was critical to figure out how the mind's thoughts became translated into actions of the body. Descartes doubted everything that was not certain and then realized that the very fact of his doubting meant he must be thinking. From there, he decided that the existence of God is also certain, and went on to define the world in terms of thought and matter, which he called extension. The mind and body interact, but the goal is to get the mind to operate solely according to reason. Alberto shows Sophie an artificial intelligence program and Sophie has a conversation with it. Major Albert Knag, Hilde's father, sneaks onto the hard drive and talks to them briefly through the computer.


Alberto then begins to tell Sophie about Spinoza. Heavily influenced by Descartes, Spinoza was the first to suggest that the Bible be read critically. He was persecuted for his beliefs, and his own family even deserted him. Spinoza viewed the world itself as a part of God. He rejected Descartes's dualism and believed that thought and extension are simply two of God's features that we can perceive. He had a deterministic view of the world, believing that God controlled all through natural laws. Spinoza felt that only God was truly free but that people could attain happiness through seeing things "from the perspective of eternity." Sophie goes to eat a banana but finds a message from Hilde's father on the peel. They determine that he is clever and powerful and Sophie suggests that perhaps he may be orchestrating all that they say. Alberto tells her not to jump to conclusions, and then he calls her Hilde as she leaves.


Sophie tells her mother a little bit about the situation with Alberto and Hilde's father, but only succeeds in making her mother worry more than ever. Two weeks go by and Sophie hears nothing from Alberto. She receives two birthday cards for Hilde. On June 14th Hermes comes to get her, and before taking her to Alberto's house he says happy birthday to Hilde. Sophie is amazed, but it seems Hilde's father can do anything. Alberto tells her about the empiricists, philosophers who felt that everything in our mind comes from our experience through the senses. They were critics of the rationalists. Locke, Berkeley, and Hume are the most important, and Alberto starts by discussing Locke. He wanted to understand where our ideas come from and how trustworthy our senses are. Locke felt we could perceive simple sensations, and that we build these up through reflection to form complex ideas. However, he also divided the world into primary and secondary qualities, and only the first—such as size or number—are accurately reproduced. Secondary qualities, like taste, vary from person to person. Locke had a few rationalistic features to his thought. He felt that the same natural rights applied to everyone and also that the existence of God was knowable through reason. Locke also advocated a division of power within government.


As Alberto teaches Sophie about each philosopher, Hilde's father does something that directly contradicts that philosopher's ideas. After learning about ##Descartes# who felt that the mind, and reason, was sacred, Albert Knag sneaks onto the hard drive of the computer and has a conversation with her. His action seems to be a blatant insult to reason, which suggests that it is impossible for someone to just sneak onto a computer's hard drive like that. While learning about Spinoza, Sophie finds a message from Hilde's father on the inside of a banana peel. Spinoza believed that God controlled the world through natural laws, and yet Hilde's father appears to play with those laws. Finally, Hermes, a dog, speaks to Sophie just before she learns about Locke and the empiricists. Yet these actions are not necessarily contradictory to the philosophy that Sophie has learned.

Part of what makes Gaarder's juxtaposition of the great philosopher's ideas with Sophie's life so powerful is the fact that their philosophies can also be made to fit with her circumstances. Although Albert Knag talking to Sophie through the computer seems ludicrous, perhaps there is a reasonable explanation. If there were such an explanation, then Descartes would probably be satisfied. For Descartes, it is not something that seems like a violation of physical reality that is a great problem, but rather something that violates reason. Descartes doubted everything in the beginning, and all that he was really sure of at first was the fact that he could doubt. Therefore, as long as whatever is happening to Sophie does not touch the fact that people have reason, it does not contradict Descartes' philosophy. Spinoza, on the other hand, was a determinist who felt that people were not truly free. Albert Knag's message on the inside of a banana peel is difficult to explain, yet who can say that we understand all of the natural laws. Spinoza was certain that the universe was rational, but there is no assurance that our reason can comprehend all. Therefore, what appears irrational to us may simply be the complex workings of the rational laws of nature. Finally, Locke, although he was an empiricist, did not believe in simply trusting everything that we perceive through our senses. Rather, he felt that only certain qualities could be perceived objectively. When Sophie heard Hermes wish Hilde happy birthday it is possible that her senses were deceiving her.

Gaarder forces us to consider more deeply the implications of the philosophy that Sophie learns. Taking into account these apparent contradictions and working them through to a resolution gives the reader both a greater understanding of the philosophies involved and also demonstrates the importance of the philosophers. Gaarder seems to be showing us that philosophy is a part of everyday life, and that the philosophers of the past will always be relevant to the present. Though it is by no means simple to grasp the importance of Descartes or Spinoza, it can be very fruitful to do so. Reading Sophie's World is supposed to make us think, and it can help us to grapple with what Gaarder believes are some of the most important questions that can be asked.