Since olden days many opinions have been expressed about which part of an opponent one should look at [during a duel] but the majority of people have supported staring at an opponent’s face. When so doing, the eyes should be narrower than usual but the mind should be broad. The eyeballs should not move and when the opponent is near they should be focused as though they were looking into the distance. In this way, a man can look at not only his opponent’s face but his whole body, thus being able to anticipate any offensive thrusts he might make.
In my opinion, there are two kinds of eyes: one kind simply looks at things and the other sees through things to perceive their inner nature. The former should not be tense [so as to observe as much as possible]; The latter should be strong [so as to discern the workings of the opponent’s mind clearly].
Sometimes a man can read another’s mind with his eyes. In fencing, it is all right to allow your own eyes to express your will but never let them reveal your mind.
This matter should be considered carefully and studied diligently
In combat many instructors have taught that you must look into the opponent’s eyes to judge their next move, but as Mushashi states there are 2 types of “eyes”, 1 that only can see the surface, while the second that can perceive the inner nature of things. The second is where a ninjutsu practitioner should always be training their perceptive abilities to, while the former is a common occurrence in new or beginner practitioners, actually some never leave this level of the martial arts throughout their training life.
Developing a set of “perceiving eyes”, involves diligent study of many aspects of combative skills, people, nature and also developing a method of quickly filtering through information rapidly. So, I leave you with this question as a first step: “When do you know someone is your opponent?”