A meditation student, while meditating in his room, believed he saw a spider descending in front of him. Each day the menacing creature returned, growing larger and larger each time. So frightened was the student, that he went to his teacher to report his dilemma. He said he planned to place a knife in his lap during meditation, so when the spider appeared he would kill it. The teacher advised him against this plan. Instead, he suggested, bring a piece of chalk to meditation, and when the spider appeared, mark an "X" on its belly. Then report back.
The student returned to his meditation. When the spider again appeared, he resisted the urge to attack it, and instead did just what the master suggested. When he later reported back to the master, the teacher told him to lift up his shirt and look at his own belly. There was the "X".