Sophie's World is both a novel and a history of philosophy, and so it is not strange that philosophy is its unifying theme. Philosophy is presented not as some esoteric exercise to be performed by people with too much free time but rather as something integral to life itself. Sophie and Alberto need philosophy to understand their world. But they are not so different from the rest of us. They can be sure that their world is the creation of Albert Knag, but just because we lack the answer to the question of where our world (or universe) comes from does not mean that we are freed from asking about it. In fact, as Gaarder stresses throughout the book, to be a philosopher is to never cease asking questions. Alberto tries to get Sophie to realize just how amazing her own existence is. It does not matter that there may be no single answer to the questions that we ask—the very asking of them is what makes us human. Why we are here, what makes a good life, and all of the other philosophical questions posed in the book are, according to Gaarder, the most important things we can ask. Once our physical well-being is taken care of we must concern ourselves with our mental lives. Life is thrust upon us, and the only way that it can mean anything to us personally is if we ask these questions constantly. Philosophy stands alone, outside of other disciplines, because in reality Gaarder equates it with living. If we live without philosophizing, then we have deprived ourselves of the greatest pleasure and understanding that we could ever come to. Philosophy is an ongoing, lifelong pursuit. We alone of all the creatures on earth can engage in philosophical reflection. Although it may not make our lives simpler or give us any easy answers, philosophy will fill us with a sense of wonder about our existence and our existence. Gaarder shows us that even when philosophy is intricately complicated, it revolves around simplicity.
The philosophical issue that plays the largest role in Sophie's World is that of free will. Sophie and Alberto learn that their existence is due to the imagination of Albert Knag. Up until that point Sophie had believed that she was an independent, free being. When they discuss the philosophy of Berkeley it becomes clear that in reality their freedom is only what Hilde's father lets them think they have. Yet, despite the fact that they are imaginary, Sophie and Alberto manage to find a way to escape. They cannot obtain what we would consider real existence, but they gain the freedom to act of the own accord. But what of Albert Knag's free will? He wrote a book for his daughter's birthday, and it seems that perhaps he was not in complete control of all that he was writing. Furthermore, some of his thoughts seem to have developed the ability to act of their own accord. Although Gaarder does not suggest that all of our actions are determined, it is also not clear the extent to which we may exercise our own free will. Perhaps there is an uncertainty in everything and even our own thoughts are not always what we want them to be. What is clear is that the concept of free will is both extremely important and very complicated to sort out.