Later that afternoon, while Sophie is thinking about philosophy, her mother finds one of the letters. Since it has no stamp, she thinks it is a love letter, and Sophie lets her think that in order to maintain her privacy. Inside it are three more philosophical questions, and Sophie puzzles over them for a day before she receives the next package. The letter tells her that her philosophy course will go from ancient Greece up to the present moment. It also points out that it is very important when assessing each philosopher to understand what his project was—what questions he was attempting to answer. Sophie learns that the ancient Greeks believed the world was eternal, and so they did not ask about where it came from but rather were interested in the question of change. The natural philosophers believed that there was one substance that all things were made of. Some thought it was water, others air, but they were all left with the problem of how changes occurred. Parmenides believed that nothing actually changed, and he held to his reason despite the evidence of his senses, making him the first rationalist. Heraclitus believed in his senses and felt that nothing stayed the same. But Empedocles resolved this problem by suggesting that there were four basic substances and that all changes are the result of intermingling of the four. He also makes a distinction between "substance" and "force", something that scientists still do today. Anaxagoras, from Athens, believed nature was made up of infinitesimal particles but that each one contained part of everything. Sophie thinks about all of this and concludes that one cannot learn philosophy; one can only learn how to think like a philosopher.
After reading the last packet, Sophie finds another white envelope in the mailbox. It asks only why the Lego is "the most ingenious toy in the world." She thinks about this question, and the next day receives a packet about Democritus, the Greek philosopher who believed that everything was made up of tiny, invisible, and eternal particles called atoms. She learns that physicists today still believe that there is some smallest particle in the physical world. Sophie is amazed by the fact that Democritus managed to use the philosophers before him to come up with a new theory.
Sophie finds another envelope with three new questions on it, and she decides to send a note of her own. She writes a letter to whomever it is who is teaching her philosophy, inviting that person to coffee. She leaves it in the mailbox and then goes upstairs to go to bed. Just before falling asleep, she thinks she sees a man in a beret come to the mailbox, put something in, and take out her letter. Sophie goes and gets the envelope and learns that the ancient Greeks were fatalists—they believed that everything in life was predetermined. However, the historians Herodotus and Thucydides and the doctor Hippocrates began to look for naturalistic explanations for the events that occur in life. The next day is Saturday, and when Sophie wakes up, she finds a scarf with the name Hilde on it.
The debates over the substance that the Greek philosophers believed made up the world are very instructive for Sophie. With Parmenides comes rationalist thought, the concept that what we perceive through our senses may be flawed, but we can trust in our reason. Change was the major problem, and Parmenides' solution—that nothing actually changes—is something that everyone feels sometimes. Although we do not deny the changes that we see occurring all around us, it often seems that things remain the same. Heraclitus, on the other hand, believes that we can only know what we perceive, and since our senses tell us that things are in a constant state of change, they therefore must be changing. The debate of whether to trust our reason or to trust our senses is one that people deal with frequently. It is fairly common for someone to not completely believe what she has seen because it goes against her reason or common sense. Empedocles' conclusion that there are four basic substances is an attempt to rectify the situation, allowing us to believe what we see but also to trust in our reason. It appears that what the philosophers were doing was attempting to explain the way the world must be based upon what we can perceive of it, and this is extremely important. Although we could live without thinking about these sorts of questions, the very questions themselves and the answers that we pose to them determine precisely what the very act of living means.
Sophie finds that everything she learns seems to make sense to her, and, moreover, to be extremely applicable to life. Philosophy is not an activity that takes place outside of everyday life; Gaarder says that nothing could be more relevant to the way we live than philosophy. In fact, in a literal sense philosophy has become a regular part of Sophie's life, since the course that she is now taking makes up a major part of her day. Science uses many of the ideas that were developed in ancient Greece, and the fact that Empedocles' distinction between substance and force is still around suggests that perhaps philosophy is really getting at something. Scientific understanding is generally regarded as telling us about the world, and since philosophy has informed our scientific understanding it suggests that what we discover through philosophical thinking may go beyond our relationship with the world to the actual features of the world itself.
The fatalism of the ancient Greeks fits in well with the mythological picture of the world. But when events in the world are no longer described in terms of the actions of gods' but rather given some sort of naturalistic explanation, then it makes sense that the events in people's lives would also cease to be attributed to supernatural causes. To believe that there is a natural explanation for change in the world but not in human life would not be very consistent, and so after taking the first steps away from a mythological worldview, it may be inevitable that fatalistic explanations of people's lives will be questioned. Sophie learns that philosophy builds upon itself, each new philosopher moving from the conclusions of those before him. But, while it changes and moves on, there is no guarantee that philosopher's answers are correct, and the same host of questions must continually be asked.