And he went back to meet the fox.

"Goodbye," he said.

"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."

"What is essential is invisible to the eye," the little prince repeated, so that he would be sure to remember.

Antoine de Saint-Exupéry,
The Little Prince