The biggest theme in this text, which reoccurrs again and again, is the need that humans have for philosophy as a means of understanding life and ordering their existence. In this novel, philosophy is not presented merely as a hobby that only those who are big thinkers or who have lots of time on their hands can practise. Instead, philosophy is shown to be absolutely vital in order for survival and understanding to occur. What Alberto tries to make Sophie realise is that her existence is wonderful, and it is the act of asking questions about our existence that makes us truly human. Note, for example, the following quote from one of the first letters that Alberto writes to Sophie:
But when these basic needs have been satisfied—will there still be something that everybody needs? Philosophers think so. They believe that man cannot live by bread alone. Of course everyone needs food. And everyone needs love and care. But there is something else—apart from that—which everyone needs, and that is to figure out who we are and why we are here.
The novel thus argues that when our basic needs have been catered to, humans should begin, and never cease, to ask ourselves these big questions about our existence and lives. Philosophy is presented as something that is necessary for humans to enjoy life to the highest degree. Philosophy is presented as a neverending journey that only humans, out of all creatures in nature, can participate. Even though asking questions might not give us the simple answers that we look for, our lives are enriched through the act of asking and thinking about those questions. This is the biggest reoccurring theme in the novel.