Many themes found in Saint-Exupery's The Little Prince depict many different principles. In fact, it is widely recognized that there isn't one social issue of Saint-Exupery's time that he neglected to touch on in the little book (eNotes, "Social Concerns/Themes"). However, the most dominant theme and also principle explored in the book is that of spiritual understanding. Time and time again, Saint-Exupery teaches that one must look beyond the corporeal world; that true understanding lies in the non-obvious, in the spiritual world that consists of things such as love and faithfulness.
We see the basic principle of spiritual, or true understanding, relayed in the very first chapter. We learn that when the pilot was a boy he drew a picture of a boa constrictor that had just eaten an elephant. He called his picture a drawing of a "boa constrictor from the outside." If you look very closely at the drawing, you'll see that it is barely recognizable as a snake if you'll notice that it has eyes. However, all of the adults he showed it to concluded that it was a drawing of a hat. Saint-Exupery uses this idea of the drawing to teach that there are those in this world, especially adults, who are too materialistic and too lacking in foresight to be able to notice anything beyond surface level. We learn that even as an adult the pilot continued to show his drawing to others and was able to judge whether or not they were capable of reaching "true understanding" by whether or not they saw the picture as a hat (Ch. 1). The ability to see beyond surface level can be likened to the ability to see beyond the corporeal and to understand the spiritual world. The little prince in our story is the only one the pilot ever met who was capable of reaching "true understanding" and seeing beyond the corporeal.
As we get to know the prince further and learn about his adventures, we also see that other basic principles Saint-Exupery touches on in the book are about love, friendship, faithfulness, conceit, drunkenness, laziness, and materialism. Hence, we see that there really are very few principles that Saint-Exupery fails to touch upon in his parable-like story.