"Impossible is a word to be found only in the dictionary of fools." - Napoleon Bonaparte
I believe that this aphorism relates to Joseph Campbell's theory of the hero's journey for one specific reason. Recall that a hero encounters many challenges throughout the story, and which he eventually overcomes. Some hero's believe that these obstacles put ahead of them are impossible to surpass. An example is young Luke Skywalker. In the first part of the Episode 4 Luke feels trapped in the jurisdiction of his uncle, but after his death, with sadness in his heart, he overcomes this obstacle. In this particular case we can see how impossible belongs to the dictionary of fools because Luke proved that nothing was that hard to overcome, and that he could in fact escape that tedious life he disliked so much. I also believe that there is always a master that teaches his pupil how to surpass his/her difficulties. In the case of Skywalker, Ben Kenobi is that master. He is trying to teach Luke that there is a way out his uncle if he learns the force.