In “ The essence of Okinawan Karate-do, Page 14, Shoshin Nagamine writes :
"Karate is self training in perfection, a means whereby a man may obtain that expertise in which there is not the thickness of a hair between a man and his deed. It is a training in efficiency, It is a training in self reliance."
In his Fukanzazengi (Eihei Koroku translation by Leighton and Okumura) Master Dogen writes
"And yet, if there is a hairbreadth of deviation, it is like the gap between Heaven and Earth, if the least like or dislike arises, the mind is lost in confusion"
In the "Hsin Hsin Ming" ("Trust in Mind") a poem from the 6th century China when Zen or Ch’an was beginning to emerge as a separate tradition, the Chinese Ancestor Seng-ts’an, or Kanshi Sosan writes :
"The Great Way is not difficult, for those who have no preferences. When freed from grasping and aversion, it reveals itself clearly and undisguised. A hair’s breadth difference, and heaven and earth are set apart. If you want it to appear, have no opinions for or against. The duality of like and dislike is the disease of the mind."
Coincidence ? I doubt it. At the end of his book, Shoshin Nagamine writes again :
I have pursued the study of Karate in an attempt to bring karate and Zen together as one.This has been a life-long effort, and one that can never be fully realized by anyone person.
Zen Masters often have written about or for Martial Arts, but less Martial Art Masters wrote about Zen.
One thing that attracted me so much to Zen is its Practical aspect. Zen is something you TRAIN in. There is nothing wrong about trying to understand some aspects of it, however, if this is your only practice, you might as well go home, you won't do anything good for yourself. But this is not clear from the outside. I believe most people consider Zen as a very intellectual and possibly bizarre philosophical or religious system. This is at least how they look at it in France.
The records we have from Martial Art Masters seldom go back more than 150 years. These masters were not living in the dark ages, but in a preindustrial age and their world was not too different from ours.
For this reason, I believe their words are easier to understand than those of people writing from 800 years or more ago. In any case, they will be easier to understand for students of Martial Arts. I know of no anthology of what Budo Masters had to say about Zen. I believe such a collection could be helpful.