“What is essential is invisible to the eye”

One of my favourite novels is “The Little Prince” by French author Antoine de Saint-Exupéry. I know that some people think it is a simple children`s tale, but to me it is far more. It is not only beautifully written, but contains some everlasting truths about friendship and love.
Take chapter 21 where the little prince meets the fox. The fox refuses to play with the little prince because, as he says, he is not “tamed”. The little prince who has come to planet Earth to find friends asks him:

“What does that mean, ‘tame’?”
“It is an act too often neglected,” said the fox. It means to establish ties.”
“‘To establish ties’?”
“Just that,” said the fox. “To me, you are still nothing more than a little boy who is just like a hundred thousand other little boys. And I have no need of you. And you, on your part, have no need of me. To you, I am nothing more than a fox like a hundred thousand other foxes. But if you tame me, then we shall need each other. To me, you will be unique in all the world. To you, I shall be unique in all the world…”
(…)
“My life is very monotonous,” the fox said. “I hunt chickens; men hunt me. All the chickens are just alike, and all the men are just alike. And, in consequence, I am a little bored. But if you tame me, it will be as if the sun came to shine on my life. I shall know the sound of a step that will be different from all the others. Other steps send me hurrying back underneath the ground. Yours will call me, like music, out of my burrow. And then look: you see the grain-fields down yonder? I do not eat bread. Wheat is of no use to me. The wheat fields have nothing to say to me. And that is sad. But you have hair that is the colour of gold. Think how wonderful that will be when you have tamed me! The grain, which is also golden, will bring me back the thought of you. And I shall love to listen to the wind in the wheat…” The fox gazed at the little prince, for a long time. “Please, tame me!” he said.
(…)
“What must I do, to tame you?” asked the little prince.
“You must be very patient,” replied the fox. “First you will sit down at a little distance from me, like that, in the grass. I shall look at you out of the corner of my eye, and you will say nothing. Words are the source of misunderstandings. But you will sit a little closer to me, every day…”
(…)
So the little prince tamed the fox. And when the hour of his departure drew near…
“Ah,” said the fox, “I shall cry.”
“It is your own fault,” said the little prince. “I never wished you any sort of harm; but you wanted me to tame you…”
“Yes, that is so,” said the fox.
“But now you are going to cry!” said the little prince.
“Yes, that is so,” said the fox.
“Then it has done you no good at all!”
“It has done me good,” said the fox, “because of the color of the wheat fields.”
(…)
“Goodbye,” said the fox. “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.”
(…)
Men have forgotten this truth,” said the fox. “But you must not forget it. You become responsible, forever, for what you have tamed.” (…)

Whole chapter here