Issai Chozanshi was the pen name for Niwa Jurozaemon Tadaaki, a samurai who lived in Japan from 1659 -1741. Little is known of his life, but he left behind several insightful works on martial arts, the most well known of which is his Tengu Geijutsuron.
It is the story of a swordsman who happens upon a group of spirits in the mountains while they are discussing the Way of martial arts. He listens in on their discussion and learns about the importance of spontaneity, acting without acting, and mushin or flowing mind.
The spirits he encounters in the mountains are called Tengu. Tengu are a kind of Japanese folk spirit that were thought to live in the rocks and the trees and could take on the shapes of men and animals. They could be harbingers of good or bad luck, and would sometimes be blamed for small acts of mischief, but they could also be depicted as wise and powerful natural forces.
In Tengu Geijutsuron the spirits discuss the importance of both technique and principle, and the discussion often returns to the importance of keeping the mind free from intention. In Chapter Two the Tengu describe the benefits of these things to the martial artist,
"Swordsmanship is the technique of contention, and it is considered essential to cut through the root of confusion of life and death from the very beginning of your study. But it is difficult to do this abruptly. For this reason, you should use all the power of your mind to cut through the principle of life and death, to develop your ch'i, to test the techniques of physical confrontation, and make great efforts without any neglect throughout this time. When you have sacrificed your life for training, when you techniques are mature, when you ch'i is under control, when principle has penetrated your mind and you have no doubts, when you are no longer confused and your spirit meets no obstruction - then your thoughts will not move within you at all. And when your thoughts do no move, ch'i will follow spirit. It will move along with animation and will flow smoothly. When ch'i rides the mind, it will not stagnate, it will not be stopped, and it will control form with complete freedom and without obstruction.
Following the perception of the mind, the speed of practical application is like opening a door and the moonlight immediately shining in; or like striking something and having the immediate response of a sound. Victory and defeat are the traces of practical application. But if you don't have conceptualization form will not have aspect. Aspect is the shadow of concept, and is what manifests form. If there is no aspect to form, the opponent you are supposed to face will not exist. This is what is meant when we say that neither my opponent nor I exist. If I exist, my opponent exists. Because I do not exist, even the insignificant thought of good or evil, perversion or properness, by the man coming at me will be reflected as in a mirror. And this is not reflected from me. It is simply that he arrives and moves on. This is just like being unable to confront with your own wickedness someone who has attained virtue. It is a mystery of the Of-Itself-So. If I tried to divert it from myself, it would become a thought. And because this thought would obstruct me, my ch'i would stagnate, and practical application would not be completely free.
The person who comes and goes like a powerful spirit neither thinking about nor enacting the unfettered mysterious function - this is the swordsman who can be said to have attained enlightenment."
In the opinion of the Tengu, it is not enough to have the proper physical skills without the proper mental ones, nor vice versa. It was only with mind and body in harmony that the warrior is able to "control form with complete freedom and without obstruction."
Then, in Chapter Three the spirits discuss the "moon in the water,"
"...there are various meanings attached to this phrase according to different schools, but fundamentally 'the moon reflected in the water' is a metaphor for when you can move and respond with spontaneity and no mind. Among the poems of the Abdicated Emperor Sotoku while he was at the palace near Hirozawa Pond is the following:
Though there is a reflection,
The moon reflects itself
Without thought, too, the water:
At the heart of this poem is an enlightened state of mind concerning action and response with no mind and spontaneity..."
The poem and the parable refer to a state of perfect emptiness, wherein our actions are instantaneous and without intention, and where the opponent is both present and not present in our minds.
The lessons of the Tengu are true today, over three hundred years since that day in the mountains. Practice with discipline. Internalize the lessons of the concepts and principals, perfect your execution of the physical techniques. And remember the moon in the water. It exists, without delay, or intention. It simply is.
Learn to fight like this, and you will understand what Chozanshi means by "Abiding in the midst of attack" and "Attacking in the midst of abidance."