Concentration is the act of focusing one's attention. As the mind is allowed to focus on a single object, it stills. As the mind is kept in the present, it becomes calm. Concentration means keeping the mind now and here. Concentration is the supreme art because no art can be achieved without it, while with it, anything can be achieved.

So when you practice zazen, your mind should be concentrated on your breathing. This kind of activity is the fundamental activity of the universal being. Without this experience, this practice, it is impossible to attain absolute freedom.

Concentration is a process of distraction from less important matters.

2. Concentration, or Yoga, is the hindering of the modifications of the thinking principle.
In other words, the want of concentration of thought is due to the fact that the mind — here called "the thinking principle" — is subject to constant modifications by reason of its being diffused over a multiplicity of subjects. So "concentration" is equivalent to the correction of a tendency to diffuseness, and to the obtaining of what the Hindus call "one-pointedness," or the power to apply the mind, at any moment, to the consideration of a single point of thought, to the exclusion of all else... Upon this Aphorism the method of the system hinges. The reason for the absence of concentration at any time is, that the mind is modified by every subject and object that comes before it; it is, as it were, transformed into that subject or object. The mind, therefore, is not the supreme or highest power; it is only a function, an instrument with which the soul works... (Book I, Concentration)

In concentration, the consciousness is held to a single image; the whole attention of the Knower is fixed on a single point, without wavering or swerving.

Attention is not concentration. When you concentrate, as most people try to do — what takes place when you are concentrating? You are cutting yourself off, resisting, pushing away every thought except that one particular thought, that one particular action. So your concentration breeds resistance, and therefore concentration does not bring freedom. Please, this is very simple if you observe it yourself. But whereas if you are attentive, attentive to everything that is going on about you, attentive to the dirt, the filth of the street, attentive to the bus which is so dirty, attentive of your words, your gestures, the way you talk to your boss, the way you talk to your servant, to the superior, to the inferior, the respect, the callousness to those below you, the words, the ideas — if you are attentive to all that, not correcting, then out of that attention you can know a different kind of concentration. You are then aware of the setting, the noise of the people, people talking over there on the roof, your hushing them up, asking them not to talk, turning your head; you are aware of the various colours, the costumes, and yet concentration is going on. Such concentration is not exclusive, in that there is no effort. Whereas mere concentration demands effort.

The art of concentration is a continual letting go. We let go of what is inessential or distracting. We let go of a thought or a feeling, not because we are afraid of it or because we can’t bear to acknowledge it as a part of our experience; but, because it is UNNECESSARY.

We say concentration, but to concentrate your mind on something is not the true purpose of Zen. The true purpose is to see thing as they are, to observe things as they are, and to let everything go as it goes. This is to put everything under control in its widest sense.

How are we to know that the mind has become concentrated? Because the idea of time will vanish. The more time passes un-noticed the more concentrated we are . . . All time will have the tendency to come and stand in the one present. So the definition is given, when the past and present come and stand in one, the mind is said to be concentrated.” — Vivekananda

Most of us in our thinking are wandering from this to that to the other thing, and are constantly distracted. And Zen is the opposite of that. It’s being completely here, fully in the present. And you know when you’re completely concentrated, you’re not really aware of your own existence. It’s rather the same as the sense of sight. If you see your eyes, that is to say if you see spots in front of your eyes, or something on the lens of the eye, then you’re not seeing properly. To the degree to which you’re seeing properly, you’re unaware of your eyes. In the same way, if your clothes fit well, you’re unaware of them on your body. And if you’re completely concentrated on what you’re doing, you’re unaware of yourself.

Concentration is a form of exclusion, and where there is exclusion, there is a thinker who excludes.

Apply the blacksmith's homely principle when you are speaking. If you feel deeply about your subject you will be able to think of little else. Concentration is a process of distraction from less important matters. It is too late to think about the cut of your coat when once you are upon the platform, so centre your interest on what you are about to say — fill your mind with your speech-material and, like the infilling water in the glass, it will drive out your unsubstantial fears.

When one cultivates strong concentration by means of tranquility or insight meditation, the mind is no longer distracted by thoughts and other hindrances. Such pure concentration, continuously focused on an object for either tranquility or insight, is considered mental purification. A mind associated with such concentration is also purified of hindrances due to the power of the concentration.

While you are continuing this practice, week after week, year after year, your experience will become deeper and deeper, and your experience will cover everything you do in your everyday life. The most important thing is to forget all gain
ing ideas, all dualistic ideas. In other words, just practice zazen in a certain posture. Do not think about anything. Just remain on your cushion without expecting anything. Then eventually you will resume your own true nature. That is to say, your own true nature resumes itself.