There is a common misunderstanding. I believe, that will hinder zazen (meditation): the idea that it is an
exercise in concentration.
This misunderstanding is common enough that my statement may sound controversial at first. And naturally we do often use words like "concentration" and "focus" when describing the beginning practice of zazen. But there is an important distinction between the usual meaning of concentration, and the true concentration that must manifest in genuine Zen meditation.
According to the usual meaning, we can say that two things are required for concentration: the subject who concentrates, and the object of concentration. If we are discussing meditation specifically we might even say that three things are required, by adding the method used to concentrate on the object, for example susokukan (breath counting).
With this kind of common concentration, many beginners take up zazen. Having been told that they should, for example, count their breaths with great focus, they set about doing precisely that. With effort, they gather up their minds and concentrate entirely on their exhalations, attempting to watch each one with a seamless continuity of attention. They are cautioned not to count an exhalation and then allow the mind to lapse into distraction; rather, they are directed to count the entire thread of each exhalation, as if intently watching a ball which has been thrown into the distance.
This is all fine, and a useful way to begin. However, it is not yet Zen at all. The reason, of course, is that the common kind of concentration is an inherently dualistic exercise: the watching of an object, the breath, by a subject. Furthermore, it is really difficult to maintain such dualistic concentration without exhaustion and cloudiness. Now, if we have been able to have the experience of recognizing our nature, kensho, through the teacher's direct pointing or other means, then naturally our practice could be different than I have just described. We may count the breaths from within that recognition, that is, giving rise again and again to it. But many students do not have such a clear recognition immediately. So here I am speaking in terms of practice before kensho.
This is the place where the physical details of zazen are crucial. Should a student simply do the method described above using the mind alone, with little physical engagement other than painfully sitting hour after hour in the formal zazen posture, little progress will be made even after several years. However, additional instruction is given orally by the teacher according to need, regarding how one cultivates the breath and senses to unify body and mind. Samadhi - meditative absorption - is thus able to manifest, and the student will experience that the separation between meditator-as-subject and breath-as-object becomes less rigid, or drops away for a time. At such a moment, we can say that real breath-counting has begun.
In other words, the common kind of concentration - that is, "concentrating on" something - is not really a factor in this practice at all. The goal is not actually to count the breath or concentrate on it. Rather, the student must manifest with the body a practice of "becoming one with" the count. Body, breath and the method must unify. When the divisions between inside and outside, meditator and object of meditation, are seen through, then for the first time it is genuine practice.
Of course, in general it is the teacher's job to impart the meaning of real zazen using words, the body and other means. A qualified teacher will aim to change the student's state using his or her own, meaning that the experience of integrated samadhi may arise spontaneously within the student, providing an important basis for the subsequent practice. There is not much that can be said about that here. But in terms of words, when teaching the breath-counting method I no longer tell students to "count the breath." Instead I tell them to "breathe the count." There is a subtle difference they may catch from this way of describing it, I think.
It is good to remember that the enso, the circle, is a symbol of Zen teaching and practice. It is not split into two sides, one of which concentrates on the other. We can say it is "one," though this one is not some thing separate from another "one." Instead we just say it is the realization of "not-two." Our breath-counting, or the way in which we kufu (work with our whole existence) to enter into a koan, must be like this.
So I would like to ask my own students to stop concentrating so hard. As Dogen said, "just throw body and mind into the house of the Buddha." Which is to say, breathe the count with your whole body, become the koan with your whole body, become the zendo as you enter, become the person as you speak to them, become the cold or the hot, the pain or the pleasant feelings that arise. In meditation, don't concentrate rigidly on anything, letting your mind fixate and stop upon it until you are exhausted. Relax completely, and dissolve into your meditation method completely with your body.
This requires a certain kind of faith. But if practice is correct, that faith will naturally arise too. So there is nothing to worry about at all.