In the 7th century CE, the Fifth Patriarch was aging and felt it was time to bestow the Patriarchal robes to his successor. In order to find his successor, he instructed the monks at his temple to submit to him a verse describing their understanding of the Dharma. The one who conveyed a true understanding would become his successor. Late that night, the head monk at the temple knew that the other monks would not submit verses until after he had submitted one, so he wrote the following verse on the central section of the south corridor in the temple:
The body is the Bodhi tree,
The mind is like a clear mirror.
At all times we must strive to polish it,
And must not let the dust collect.
The Fifth Patriarch read this verse the next morning, and after having learned that the head monk wrote the verse, said to him, "This verse you wrote shows that you still have not reached true understanding. You have merely arrived at the front of the gate, but have yet to be able to enter it. If common people practice according to your verse they will not fall. But in seeking the ultimate enlightenment, one will not succeed with such an understanding." He then instructed the monk to meditate for several days and write another verse. If the monk's new verse showed that he had seen his own original nature, he would earn the Robes. The monk was unable to write another verse.
Several days later, the illiterate cook Hui-neng, who had been working at the temple for eight months, noticed the verse on the wall, and asked an acolyte to read it to him. After hearing the head monk's verse, Hui-neng recognized that the author of that verse had yet to know his true nature, and had the acolyte write the following verse on the same wall:
Bodhi originally has no tree,
The mirror also has no stand.
Buddha nature is always clean and pure;
Where is there room for dust?
After reading this new verse, the Fifth Patriarch expounded the Diamond Sutra to Hui-neng who was immediately awakened, and bestowed the Patriarchal Robes upon him. (Platform Sutra of the Sixth Patriarch, trans. by P.B. Yampolsky)
While the head monk's verse does not display a true understanding of his nature, the practice he expounds is the base of all our training.
Ethics. Concentration. Wisdom. Build a strong base; polish your mind. And know that Bodhi originally has no tree.