If a student's mind has become stable through zazen, the application of the koan may generate the Great Doubt. This doubt is not the ordinary doubt that questions the truth of an assertion. It refers to the practitioner's deeply questioning state of mind which results from investigating the koan. In fact, the resolution of the koan hinges on the nurturing of the Great Doubt. Because the meditator cannot answer his question by logic, he must continually return to the question itself, and this process clears the mind of everything except the Great Doubt The “doubt mass” that accumulates can disappear in one of two ways. Due to lack of concentr ation or energy, the meditator may not be able to sustain the doubt, and it will dissipate. But if he persists until his doubt is like a “hot ball of iron stuck in his throat,” the doubt mass will burst apart in an explosion.

If that explosion has enough energy, it is possible that the student will become enlightened. A master is needed to confirm the experience since the student, with rare exceptions, cannot do that himself. Even as great a figure as Ta-hui did not penetrate sufficiently on his first experience. His master Yuan-wu K’o-ch’in (1063-1135) told him, “You have died, but you haven’t come back to life.” Ta-hui was confirmed on his second enlightenment experience. Without the guidance of a genuine master such as Yuan-wu, Ta-hui may have settled unwittingly for a partial realization.

In the early twelfth century, Ch’ang-lu Tsung-tse wrote the Manual of Zazen (Tso-ch’an i). He insisted that a person who has experienced Buddha-nature should continue to practice zazen. Then one can become like a dragon who gains the water, or a tiger who enters the mountains. A dragon gaining the water returns to his ancestral home, free to dive as deep as he wishes. A tiger entering the mountains has no opposition; he may ascend the heights and roam at will. Thus Zen teaches that