the greatest advocates of kung-an practice. From his
　　   record of sayings we see that he maintained that tso
　　   -ch'an was very necessary to  settle   the  wandering
　　   mind, and bring about emergent samadhi. It   is  only
　　   then that the student can effectively use the   kung-
　　   an or hua-t'ou. Even though kung-an and hua-t'ou
　　   practice can be done while walking, standing, or  even
　　   lying down, its fundamental basis is still tso-ch'an.
　　  
　　   页384
　　  
　　　　  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"  疑团
　　   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  圆悟克勤
　　   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.