{T]he application of the
kung-an or hua-t'ou [a single phrase from a Koan like "Mu"] may cause the rising of the Great
Doubt ???. 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" ??
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
expollution. 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 ????
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.
http://ccbs.ntu.edu.tw/FULLTEXT/JR-BJ001/02_10.htm