Do what you wish

Bastian had shown the lion the inscription on the reverse side of the Gem. "What do you suppose it means?" he asked. "DO WHAT YOU WISH. That must mean I can do anything I feel like. Don't you think so?" 
     All at once Grograman's face looked alarmingly grave, and his eyes glowed.
     "No," he said in his deep, rumbling voice. "It means that you must do what you really and truly want. And nothing is more difficult."
     "What I really and truly want? What do you mean by that?"
     "It's your own deepest secret and you yourself don't know it."
     "How can I find out?"
     "By going the way of your wishes, from one to another, from first to last. It will take you to what your really and truly want."
     "That doesn't sound so hard," said Bastian.
     "It is the most dangerous of all journeys."
     "Why?" Bastian asked. "I'm not afraid."
     "That isn't it," Grograman rumbled. "It requires the greatest honesty and vigilance, because there's no other journey on which it's so easy to lose yourself forever."
     "Do you mean because our wishes aren't always good?" Bastian asked.
     The lion lashed the sand he was lying on with his tail. His ears lay flat, he screwed up his nose, and his eyes flashed fire. Involuntarily Bastian ducked when Grograman's voice once again made the earth tremble: "What do you know about wishes? How would you know what's good and what isn't?"

Michael Ende - The Neverending Story