But it's not just learning things that's important. It's learning what to do with what you learn and learning why you learn things at all that matters.
The princess of Rhyme tells this to Milo when he finally reaches the Castle of the Air in Chapter 18. Milo explains that if he had not made so many mistakes, he would have gotten there sooner. The princesses, of course, are pleased by his eagerness and good nature, and they tell him that mistakes are perfectly all right, so long as Milo has learned from them. They then elaborate on learning, illustrating the importance Juster places on this theme. Milo's education in the Lands Beyond can be broken into two phases. In the first phase, Milo encounters teachers of sorts who dispense lessons that are commemorated with gifts. In the second phase—which Milo has just completed when he reaches the Castle in the Air—the lessons must be put to use. Milo uses what he has learned to defeat a host of demons and comes to appreciate even more the knowledge he has gained.