By oneself, indeed, is evil done; by oneself is one defiled. By oneself is evil left undone; by oneself, indeed, is one purified. Purity and impurity depend on oneself. No one purifies another.
Rinzai zen. A response to the torpor of monks who apparently practiced in cloistered conditions, achieving perhaps a state of samadhi but without satori or kensho, the hallmark of Zen.
Koan practice. A formalized version of the quest that is the heart of Buddhist practice, the quest of those who seek deliverance from beyond the comforts and confines of the familiar, of home or God, from beyond the rituals and/or austerities of traditional forms of prayer and other practices, beyond even the morality and righteousness of religion as a bona fide public institution. On this path, nothing and no one frees you, not even God. The quest is a solitary one and deliverance is by oneself alone.
But when the practice crystallizes into a belief: Zen. Or into a style of practice: zazen. Or into a method of instruction: koan practice and dokusan. Here is the risk of becoming a disciple emulating a venerated teacher or tradition, grasping at that je ne sais quoi you are striving so hard to attain but only going through the motions of transformation and freedom. You are trying to be someone. So the content of practice reifies into the form, the quest hardens like a stone into a rote question, and surrender turns into the bones and sinew of the will to go on rather than the courage to yield.
And so there may come a time to leave; to go into the dark, alone, perhaps a violent act of freedom that breaks expectations like a fist shattering a sheet of ice. No person, no light to guide you. Light breaking through the darkness nonetheless:
Tokusan asked Ryutan about Zen far into the night.
At last Ryutan said, "The night is late. Why don't you retire?"
Tokusan made his bows and lifted the blinds to withdraw, but he was met by darkness. Turning back to Ryutan, he said, "It is dark outside."
Ryutan lit a paper candle and handed it to him.
Tokusan was about to take it when Ryutan blew it out.
~ The Mumonkan, case 28