A monk decides to meditate alone, away from his monastery.
He takes his boat out to the middle of the lake, moors it there, closes his eyes and begins his meditation.
After a few hours of undisturbed silence, he suddenly feels the bump of another boat colliding with his own.
With his eyes still closed, he senses his anger rising, and by the time he opens his eyes, he is ready to scream at the boatman who dared disturb his meditation.
But when he opens his eyes, he sees it's an empty boat that had probably got untethered and floated to the middle of the lake.
At that moment, the monk achieves self-realization, and understands that the anger is within him; it merely needs the bump of an external object to provoke it out of him.
From then on, whenever he comes across someone who irritates him or provokes him to anger, he reminds himself, "The other person is merely an empty boat. The anger is within me."