If through tso-ch'an a student's mind has become very peaceful and stable, the application of the kung-an or hua-t'ou may cause the rising of the Great Doubt 大疑情.This doubt is not the ordinary doubt of questioning the truth of an assertion. It is the doubt that arises out of ts'an Ch'an, investigating Ch'an. It refers to the practitioner's deeply questioning state of mind as a result of using the kung-an or hua-t'ou. The resolution of the kung-an or hua-t'ou hinges on the nurturing of the Great Doubt. Because the answer to his questions cannot be resolved by logic, he must continually return to his question, and in the process, clear his mind of everything else except the Great Doubt.

Eventually, this accumulated "doubt mass" 疑團 can disappear in one of two ways. One way is that, due to lack of concentration or energy, the meditator will not be able to sustain the doubt, and it will dissipate. Another way is that by persisting until his doubt is like a "hot ball of iron stuck in his throat", the doubt mass will disappear in an explosion.

If the explosion has enough energy, it is possible that the student will experience "Ch'an", see Buddha-nature, become enlightened. If not, there will probably still be some attachment in his mind. It is necessary for a master to confirm his experience, since the student, with rare exceptions, cannot do that himself. Even as great a master as Ta-hui did not penetrate sufficiently on his first experience. His master Yuan-wu K'e-ch'in 圓悟克勤 told him, "You have died, but you haven't come back to life." He was confirmed on his second experience. So what is a true experience? It takes an adept master to tell. If he is not a genuine master, he won't know the difference.

Tso-ch'an After Enlightenment