To strike your opponent, you need to enter through a gap in his defenses. If your opponent’s defenses are sound, you cannot enter, and you cannot strike him. There is a variety of ways to enter. You can overpower the defenses. You can deceive your opponent into misplacing his defense and so take advantage of an opportunity where he is exposed. You can perceive a weakness in your opponent’s posture or awareness.
Your opponent will regard your defenses the same way.

In medieval Japanese budo, these openings in the opponent’s defenses, these opportunities to strike, were called “suki.”

A medieval Japanese text on the subject, published as one of the essays in “The Unfettered Mind,” was written as a letter by Zen monk Takuan Soho to Yagyu Munenori, sword master to the family of the Shogun, the military dictator of Japan at that time.

Takuan was an influential person, abbot of one of the chief Zen temples in Japan. He was equivalent in influence to a Pope in medieval Europe. He was employed by the military government. He addressed his comments to the senior instructor in the most prestigious military art in the highly militarized world of his time.

His comments were ostensibly a discourse on swordsmanship, but can easily be read as advice in Zen practice, using swordsmanship as a metaphor. In his letter, he applies Zen insights and theory to the practice of swordsmanship. Takuan did not practice swordsmanship himself. He may have been motivated by a desire to persuade the leaders of his nation of the practical utility of Zen in the life of the samurai.

Takuan talks about avoiding “suki” by means of the “mind abiding nowhere.” This is an application of the theory underlying the Zen practice of the cultivation of mental stability and clarity in seated Zen meditation to the practice of combat with swords.

D.T. Suzuki, one of the leading importers of Zen to the West in the early 20th century, cites Takuan’s letter and analyzes it, in his book “Zen in Japanese Culture.” Suzuki was a scholar, trained in Western philosophy, but also in the practice and theory of Japanese Zen.

Both Takuan and Suzuki emphasize that the mind that is completely free to respond unhindered to the demands of the moment can also be described as the mind that is immovable — not deflected by either stimulation or impulse.

Both Takuan and Suzuki seem to take for granted that a suki arises as a result of a mental flaw — a gap in attention or alertness. As acute as their observations are, this presumption is one-sided, probably due to the fact that these two commentators were not martial practitioners and had a psychological bias to their analytical approach.

Less-experienced fighters seem to go to the other extreme. In a sparring match or a confrontation, these individuals look for a physical “gap” in the defenses of the opponent. Waiting for a physical gap to open up, their fleeting opportunity is lost. They lose initiative and fall into a reactive mode, in which they respond to their opponent’s initiative and lose any advantage they may have had.

Their analytical prejudice is physical — they neglect other aspects of combative engagement.

Today in martial arts and police combative training, we work with the same set of tactical considerations as medieval Japanese martial artists did. The opportunity to take advantage of a suki, or gap, in an opponent’s defense, and at the same time avoid a window of vulnerability to open in our own defenses, arises in three dimensions of experience simultaneously: time, space and mind.

The dimension of mind has several aspects. One aspect is alertness. This is a cultivated ability to maintain a clear and stable focus on the matter at hand, without being distracted by parts of reality such as the opponent’s body, by elements of the environment, or by one’s own thoughts, emotions and impulses. This ability is developed. You have to practice stability and clarity under pressure, just as you have to practice physical technique, to increase your strength, speed, endurance, balance, focus, etc.

Another is will: the intention to take the initiative and prevail in the encounter. This aspect of mind is also developed through training.

Another aspect of the dimension of mind, however, is not trained, but inherent. Although it can be modified slightly, it cannot be eliminated. So if we are aware of it, and know how to use it, we can take advantage of this flaw in our opponent’s ability to respond to our attack. This aspect of mind is called the “reactionary gap.”

An unanticipated arm’s length attack is almost impossible to stop. That is why, as a police officer, you are taught not to permit a subject within your reactionary gap. The reactionary gap can be described in space, as a distance of six feet from an empty hand opponent, let’s say, or it can be described in time, as 300 to 500 milliseconds for a normal person, to perceive an opponent’s motion, interpret it and respond to it.

A major league baseball player has a reactionary gap – an ability to perceive the path of travel of the pitch and respond to it by swinging the bat – of about 100 to 150 milliseconds.  This is much faster than an untrained person could respond, but still slow enough for a skilled pitcher to get a well-thrown breaking ball past him.

A famous test of the reactionary gap in armed encounters is called the Tueller Principle. The idea came out of a court case in which an officer shot a knife-wielding subject at what seemed to the jury to be a great distance. The defense contended that the distance was so close as to leave the officer no choice but to shoot or die. They tested their theory.

The test showed that a knife-wielding subject could close the distance on an officer with a holstered gun and kill the officer before that officer could draw his gun, from a distance of 21 feet or greater. And that was an average subject and an average officer. Given a subject who is a fast runner, or an officer whose attention is divided or is slower in drawing his firearm, that reactionary gap can open to 30 or 40 feet or more.

The effectiveness of any response to threat stimulus will depend on the quality of the attention of the responder, and on the habit of responding to attacks accurately, with strength, skill and speed, under pressure.

We will respond, successfully or otherwise, in the dimensions of time, space and mind.

When Zen-inspired Japanese martial arts literature describes becoming “one with the opponent,” they are not recommending that you become the same as that person, indistinguishable from him, or with the same objectives or methods as him.

The advice given in that poetic phrase is to close the gap between your mind and his mind, between your body and his body. With practice, you can intuit how quickly he can respond; you can sense positions in which you can close the distance to him and execute a technique in an unexpected way; you can feel when you can enter his reactionary gap without opening up one of your own.

This does depend on taking the initiative. But it does not mean plunging heedlessly in like a kamikaze.

I worked with a group of new trainees who were asked, on the first day of training, to take one of their instructors to the ground. The instructors were skilled. Although the trainees may not have thought of “finding a gap,” it was evident that the trainees could not find a gap in their instructor’s defense.