"During the period when the wanderer was practicing Zen meditation, his master, one day, went to one comer of the dojo (the, site where his disciples congregated), and retumed with a bamboo switch. Some of his students -- those who had not been able to concentrate well -- raised their hand. The master approached each of them and delivered three blows with the switch on each shoulder.
When he saw this for the first time, the wanderer regarded it as medieval and absurd. Later, he understood that it is often necessary to transfer spiritual pain to the physical plane in order to perceive the evil that it creates. On the road to Santiago, he had leamed an exercise that involved the digging of the nail of his index finger into the skin of his thumb whenever he had thoughts that were critical of himself.
The terrible consequences of negative thoughts are perceived only much later. But by allowing such thoughts to manifest themselves as physical pain, we are able to understand the damage they cause. And then we are able to banish them."