At the end of Sophie's World we see the theme of existence again, a theme concerned with the meaning of life and of understanding mankind's place in the universe. This is a theme that is returned to again and again, for example in the conversation after the celebration when Knag and Hilde sit on the glider and talk about the universe and the enormous expanse it encompasses. The major tells Hilde about the immense vastness of the universe in terms of the time it takes for the light from distant stars to reach our planet. They discuss the fact that a star is already old and in the past by the time we see it, perhaps even by millions of years.
The major describes the Big Bang, in which all matter in the universe converged and then exploded. He explains that the stars and planets and other matter in the universe are now moving outward and wonders if it will continue expanding or converge again.
Watching the discussion are Sophie and Knox. He points out that it is ironic that they are all watching each other without necessarily knowing it. This again relates to themes of existence, time and space. Sophie hits Hilge's head with a wrench to make her notice but Hilde only feels it stinging. Then Sophie and Knox have trouble with the boat when they want to go out and row as they can't untie it. The boat gradually unties and Sophie sees it floating way out and on to the lake. She and her dad decide to swim out to get it. So at the end of Sophie's World, the book that Hilde has been studying, Hilde's father returns and they discuss it. Hilde decides that Sophie continues to exist in some form out in the universe. This confirms the theme as the meaning of existence because Sophie and Alberto have a new existence as spirits. Both have escaped from the clutches of Albert and are no longer trapped inside his mind. However no one else can see them and, like ghosts or unseen forces, they can pass through other people. This reminds us of the latest theories of physics involving time, matter and space and the universe theme. Another theme is power or control as Sophie would like to have influence in the abstract new world of Hilde. In the last chapters we see her trying to find out how to do this as her attitude changes.