Introspecting the koan
Dahui is regarded today as the great champion of the koan method, and he was celebrated during his life for a running disagreement he had with the Caodong (later Soto) school. In a sense, this dispute drew the distinctions that still divide Zen into two camps. The issue seems to have boiled down to the matter of what one does with one's mind while meditating. The Caodong masters advocated what they called Silent Illumination, which Dahui preferred to call Silent Illumination Heterodox. The Caodong masters believed that enlightenment could be achieved through sitting motionless and slowly bringing tranquility and empty nonattachment to the mind. The koans were recognized to be useful in preserving the original spirit of Chan, but their brain-fatiguing convolutions were not permitted to disturb the mental repose of meditation.
Dahui, in contrast, believed that this silent meditation lacked the dynamism so essential to the sudden experience of enlightenment. His own-approach to enlightenment came to be called Introspecting-the-Koan, in which meditation focused on a koan.
Another of Dahui's objections to the Silent Illumination school seems to have been its natural drift toward quietism and the divorcing of men from the world of affairs. This he believed led nowhere and was merely renouncing humanity rather than illuminating it.
These days there [rival masters] whose own eyes are not clear, who just teach people to stop and rest and play dead. They teach people to "keep the mind still," to "forget feelings" according to circumstances, to practice "silent illumination.". . . . To say that when one has put things to rest to the point that he is unawares and unknowing, like earth, wood, tile, or stone, this is not unknowing silence—this is a view of wrongly taking too literally words that were (only) expedient means to free bonds.
He seemed to be counseling never to forget that meditation is only a means, not an end. Instead Dahui advocated meditating deeper and ever deeper into a koan, focusing on the words until they "lose their flavor." Then finely the bottom falls out of the bucket and enlightenment hits you.
This "Introspecting the Koan" form of Chan became the standard for Rinzai Zen, whose students were encouraged to meditate on a koan until it gradually infiltrated the mind. As described by Dahui:
Just steadily go on with your koan every moment of your life. . . Whether walking or sitting, let your attention be fixed upon it without interruption. When you begin to find it entirely devoid of flavor, the final moment is approaching do not let it slip out of your grasp. When all of a sudden something flashes out in your mind, its light will illumine the entire universe. . . .
The important thing is to concentrate totally on a koan. This concentration need not necessarily be confined to meditation, as Dahui illustrates using one of the more celebrated one-word statements of Yunmen.
A monk to Yunmen: "What is Buddha?"
Yunmen: "A dry piece of shit."
Thomas Cleary comments:
Just bring up this saying. . . Don't ask to draw realization
from the words or try in your confusion to assess and explain
. . . .Just take your confused unhappy mind and shift it onto "A
dry piece of shit." Once you hold it
there, then the mind
... will naturally' no longer operate.
When you become aware
that it's not operating, don't be afraid of falling into emptiness.
. . . In the conduct of your daily activities, just always let
go and make yourself vast and expansive. Whether you're in quiet
or noisy places, constantly arouse yourself with the saying "A
dry piece of shit." As the days and months come and go,
of itself your potential will be purified and ripen. Above all
you must not arouse any external doubts besides: when your doubts
about "A dry piece of shit" are smashed, then at once
doubts numerous as the sands of the Ganges are all smashed.
Kungan and huatou
Modern-day Chan celebrates Dash's advocacy of huatou practice. A huatou (literally, "head of a spoken word") is a question that a practitioner asks himself or herself. For example, "What is Wu?" and "Who am I?" Huatou is sometimes spoken of as a “short cut” method of working with koans that emphasizes seeing into the main point of a koan while not delving into its subtler details.
While the legends around Dahui's rivalry with Caodong school and its silent illumination are well-known, Dahui, in fact, advocated quieting the mind through sitting meditation as a prerequisite to haotou or kungan practice. A scattered mind lacks the focus or energy necessary to generate the great doubt, while in a stable and concentrated mind the application of kungan or huatou can cause the great doubt to rise.