This pass-or-fail technique differentiated Ch'an from all previous Buddhist sects; Ch'an allowed for no gradual progress upward in the spiritual Therarchy through the mastery of rituals. In the early days of the Tang dynasty, when the number of initiates was small, the great masters of Ch'an directly tested the non-rational understanding of novices; in the later years of the Sung dynasty it was necessary to develop a more impersonal procedure, such as handing out the same _koan _to a number of novices during a lecture. The more effective exchanges between the old T'ang masters and their pupils began to be reused by later teachers in the Sung, who had neither the genius to create new challenges for their novices nor the time to tailor-make a special problem for each new face appearing at the monastery. Out of this there was gradually canonized what are now the classic _koan _of Zen. Late in the Tang and early in the Sung period the _koan_ themselves began to be written down and used as the scriptures, resulting in a catalog said to number around seventeen hundred today. The _koan_ is a uniquely Zen creation, a brilliant technique developed by the T'ang masters for transmitting a religion which revered no scriptures and had no god. It appears nowhere else in the vast literature of world mysticism. Several of the greatest masters of the T'ang developed their own schools of Ch'an, and the two most successful--the Lin-chi (Japanese Rinzai) and the Ts'ao-tung (Japanese Soto)--were later transmitted to Japan. The Rinzai school pursued a technique of "sudden" enlightenment; the Soto school, "gradual" enlightenment. These terms can be misleading, however, for sudden enlightenment may require more time than gradual. The gradual school taught that by sitting in meditation (Japanese _zazen_) for long periods of time--kept awake by thrashings if necessary--one's mind slowly acquires a detachment from the world of false reality perceived by one's discriminating senses and thus achive enlightenment. It is a slow, cumulative process. By contrast, the sudden school de-emphasizes _zazen _in favor of study of _koan_. The student struggles with _koan_, building up a kind of hopeless tension which may last for years, until at last his logical processes suddenly short-circuit and he attains enlightenment. Practitioners of the sudden school also use shouts and beatings to jolt novices out of their linear, sequential thought patterns. Students of the gradual school are also invited to study _koan_, and those in the sudden school are encouraged to practice _zazen_, but each school believes its own approach is best.