Characters in The Little Prince, especially the prince, are really committed to learning what is true. “True,” in this sense, doesn’t just mean what’s actual versus what’s a lie, but what is real or as the fox puts is, “what is essential” (21.36). In other words, what is the essence of a person or thing? What is important?
You can claim you own the stars by counting them, like the businessman does, but that doesn’t mean you really own them. So the businessman might not be a liar, but he is pursuing a lie. And because he is driven by greed, he is blind to the essential and is focusing on unimportant things.
Sometimes, the truth isn’t what you see: it’s what you can’t see. The fox points out that by closing your outer eyes, you open your inner eyes to the truth. For instance, the prince thinks his flower is just like the hundreds of other roses he sees on earth—but then he learns to see her with his heart and understands the truth about their relationship. This is what the fox tries to teach the prince, and what the prince tries to teach the narrator.
The story of The Little Prince shows that people do find the truth by seeking it with their hearts and minds, rather than just paying attention to what they can see on the surface.
While it’s tempting to believe the fox, at the end of the day we can’t prove anything he says: his advice sounds beautiful but is impractical.