A monk decided to meditate alone, away from his monastery. He took his boat out to the middle of the lake, moored it there, closed his eyes and began meditating. After a few hours of undisturbed silence, he suddenly felt the bump of another boat colliding with his own.
With his eyes still closed, he felt his anger rising, and by the time he opened his eyes, he was ready to scream at the boatman who had so carelessly disturbed his meditation. But when he opened his eyes, he was surprised to find that it was an empty boat that had struck his own. It had probably gotten untethered and floated to the middle of the lake.
At that moment, the monk had a great realization. He understood that the anger was within him; it merely needed the bump of an external object to provoke it out of him. From then on, whenever he came across someone who irritated him or provoked him to anger, he would remind himself, that the other person was merely an empty boat, the anger was within him.