From the book Emotional Structure – Creating the Story Beneath the Plot by Peter Dunne
The mentor represents the protagonist’s highest aspirations.
He personifies the kind of moral person the protagonist wishes he could be and mirrors the protagonist’s spiritual center. While the mentor is allowed to give the protagonist all the encouragement in the world, he isn’t allowed to give him any answers. And the reason is simple. The mentor’s answers are HIS answers. The protagonist has to find his own.
Learning how to find the answers is the lesson being taught.