Zen Master Bankei Yotaku composed the following instructions on the art of combat for his disciple and patron Kato Yasuoki, daimyo of Ozu and an expert in the use of the yari, or Japanese lance.
Here Bankei expresses the importance of Mushin (No-mind) in combat. In the middle of the confrontation, one should not let any thought arise, actions should not be driven by a reflection or personal emotions. You should not act, but let the action simply happen through you. As soon as your mind comes in the way of action, your opponent - if he is worthy - will be able to foresee you.
In performing a movement, if you act with no-mind, the action will spring forth of itself. When your ki changes, your physical form changes along with it. When you're carried away by force, that is relying on "self." To have ulterior thoughts is not in accordance with the natural. When you act upon deliberation, you are tied to thought. The opponent can then tell [the direction of] your ki. If you [try to] steady yourself by deliberate effort, your ki becomes diffuse, and you may grow careless. When you act deliberately, your intuitive response is blocked; and if your intuitive response is blocked, how can the mirror mind appear?
When, without thinking and without acting deliberately, you manifest the Unborn, you won't have any fixed form. When you are without fixed form, no opponent will exist for you in the whole land. Not holding on to anything, not relying one-sidedly on anything, there is no "you" and no "enemy." Whatever comes, you just respond, with no traces left behind. Heaven and earth are vast, but outside mind there is nothing to seek. Become deluded, however, and instead this mind becomes your opponent. Apart from mind, there is no art of combat.
From the Book : Bankei Zen: Translations from the Record of Bankei
Bankei Yōtaku (盤珪永琢, 1622-1693), the son of a Ronin Samurai turned Doctor, was a very popular and influential teacher who spoke directly, avoiding sutras, koans and rituals.
He talked to huge crowds of ordinary people and advanced Zen students all the same, about what he had personally discovered through his own experience—"the Unborn" or "the Birthless Buddha-mind".
Expressed in a plain, simple and direct language that anyone can understand, Bankei's Zen is refreshingly clear and relatively simple. You don't have to be learned, live in a monastery or even necessarily consider yourself a Buddhist to effectively practice it.