An allegorical story

A traveler found a guy who was tied up with a tree and shouting. He asked to a villager the reason why the man was tied up. The villager answered: this man is insane and shouting all day, so we must bind him to a tree. The traveler also asked the man bound to the tree. The man bound to the tree answered: I'm not insane. The villagers bound me up with the tree without the reason, why can't I shout out for help? After that the traveler asked the villager if the bound man could be released, because his saying seemed to be reasonable. The villager sighed and said: This is why this man insane. If he was sane, he could realize that he could prove his sanity by stopping shout. However, he still keeps shouting, this proves that he is insane. We can't release him.