Although Sophie wants to hear about Berkeley, Alberto decides to discuss Hume first. He was the greatest of the empiricists and had a tremendous influence on Kant, a later philosopher. Hume was concerned with cleaning up our thoughts. He believed that perception was made up of "impressions" and "ideas". Impressions are how we experience the world, and ideas are what we recall of our impressions. Both ideas and impressions can be simple or complex, but complex ideas can be made through our imagination—an angel is one example. He wanted to examine our complex ideas in order to throw out everything that did not stem directly from impressions. He pointed out that we have no unchanging ego, since what we perceive as ourselves is in reality a huge number of perceptions that change rapidly. The Buddha also believed this, and both he and Hume opposed the idea of an eternal soul. Hume was an agnostic—he felt the question of God's existence was beyond human reason. Hume believed that what we cannot know for sure that what we call laws of nature are unbreakable. Just because every time we have seen a stone dropped it has fallen to the ground does not mean that it has to do so. We simply expect it to fall. We impose our idea of cause and effect on the world. We perceive a billiard ball hitting another and decide that the first causes the movement of the second. In reality, all we have seen is that the second moves after the first and we ascribe causality to what we have seen occur again and again. Hume also pointed out that we act in accord with our feelings, not our reason. He warned against concluding that what is is what ought to be.
A plane flies by, trailing a banner wishing Hilde a happy birthday. As black clouds appear, Alberto begins to discuss Berkeley. Berkeley questioned even more than the other empiricists. He suggested that even external reality itself may have no substance. Berkeley felt that all of our feelings and ideas can stem from our souls—just like when we are dreaming. But he also thought that all of external reality could come from another spirit. Berkeley believed that we exist only in God's mind. And Alberto thinks that they exist only in Albert Knag's mind. That is the explanation for everything that has been happening to them. He thinks that Hilde's father is writing or telling their story for his daughter's amusement. Alberto calls Sophie Hilde a few more times and then lightning flashes and Sophie runs out of the house.
Hilde Møller Knag wakes up on Friday June fifteen, excited for her birthday and eager for her father to return in a week's time. She looks outside and thinks of the time she fell overboard in the rowboat and the boat had been left floating in the middle of the bay. Hilde looks at her reflection and remembers how she used to try to wink at it with both eyes because her father said it was possible in this magic mirror. She sees a large package by her bed and grows agitated because it might be the strange present she expects from her father. Hilde opens the package and finds a ring binder filled with typed pages. The title is Sophie's World. She begins reading it. The book tells Sophie's story, and Hilde rapidly moves through the chapters. She realizes that Sophie must have been very confused by the birthday cards that were sent from her father. Hilde also wonders about her silk scarf that Sophie found—she knows it must actually be someone, not just in the book. Her mother comes in to wish her happy birthday and has a difficult time getting Hilde to look away from the book. Hilde finds the story enthralling, but also begins to get annoyed with her father for confusing Sophie, Alberto, and Joanna so much. When Sophie finds her gold crucifix Hilde becomes very confused, because she does not know how her father could have known it was lost. She becomes certain that Sophie actually exists.
Gaarder connects the idea that Sophie is a part of Albert Knag's imagination to Berkeley's philosophy. We know all along that Sophie is a character in a book, because we can read that book. Sophie's life does not continue unless we decide to read more of the book. As a result, it is impossible to ignore the possibility that the same could be true of our own lives. Although it seems improbable that we exist only in one person's imagination, we cannot know for sure. It can be fun and interesting to think of the possibility that life itself can be a dream or an active construction in someone else's mind, but Gaarder does not say how that thought affects the way we live.
Sophie's World presents us with the possibility that our existence may not really be what we believe it to be. To fully understand our existence, we can rely on the philosophers Sophie has studied. As long as what happened to Sophie does not happen to us, we can go on believing that we are not the figments of someone else's imagination. While we cannot know for sure either way, perhaps Gaarder is pointing out that it is better not to know. If we knew, like Sophie does, that our entire lives were created by some other mind and that we did not actually exist, it would be a somewhat depressing realization. On the other hand, the fact that we cannot know leads us to look at our lives in a different way. Berkeley points out that we cannot be sure even of the world. In a way, such an uncertainty only makes life itself seem more magical.
Regardless of what we conclude about our actual existence, what we learn from Hume is still critical. Hume helps us understand how much of what we think we understand about the world may be due our habit of seeing things happen the same way. We must always be receptive to new occurrences. It is not a coincidence that the chapter about Hume occurs right before Sophie discovers that she is a part of Albert Knag's mind. Hume prepares both her and us for this shock by insisting that we have a limited knowledge of the world and that just because we have seen something happen many times does not mean we can count on it happening again. In the same manner, just because we have never seen something occur does not mean it cannot occur. Hume teaches us the danger of imposing our minds on the world.