The greatest lesson in Saint-Exupery's The Little Prince is taught by the fox to the title character:
And now here is my secret, a very simple secret: It is only with the heart that one can see rightly; what is essential is invisible to the eye. (Ch. 21)
The fox's lesson serves to develop Saint-Exupery's two central themes: the importance of reconnecting with one's child self and the importance of finding or maintaining love. According to the fox, love cannot be maintained without this knowledge, knowledge that "[m]en have forgotten." Only children intuitively have the knowledge; therefore, only the child self, the one who is able to see what's invisible, truly has the ability to love.
Based on the narrator's "Drawing Number One" of the "boa constrictor digesting an elephant ... from the outside," we know Saint-Exupery wants to show that children are able to see things adults cannot see: they are able to use faith to see with their imaginations things that are not visible to the eye. This faith gets crushed in the adult world by things that are considered to be of more importance, like math and science. Yet, Saint-Exupery wants to show us that this faith is essential for a fulfilling life, and love cannot be found nor maintained without it. In finding one's faith, one is reconnecting with one's child self. In the story, the little prince reconnected with his child self by realizing how much he loved his rose and coming to understand how responsible he is for her. Likewise, the narrator reconnected with his own child self by returning to do things that were important for him, such as drawing, and through his love for the little prince.