‘In Dogen’s Fukan Zazengi (The Manner of Doing Zazen, as Recommended for Everyone), Dogen wrote about how to regulate the breath. He gives a very simple and plain instruction. He says, “Breathe quietly through your nose.” That is all. By referring to his other descriptions on how to regulate the breath, for example in “Bendoho” (The Model for Engaging the Way) of “Eihei Shingi” (The Pure Standards for the Zen Community) and in “Eihei Koroku” (The Analects of Eihei Dogen)”, I understand this instruction as this: “Let the air come in and go out quietly through your nose. Make it sure that the air is going deep down to your lower abdomen. Let your breath happen naturally (not artificially.) Do not allow your breath to become coarse or noisy, nor gasp for air. You should not experience any difficulty in breathing. Your breathing should be quiet and subtle.” When our posture is not good or our mind is agitated, it is impossible to have this “quiet, subtle and deep breath,” even if we try hard.
Another example: A variety of psychological problems during zazen, such as feelings of discomfort or instability, drowsiness or agitation, are partially caused by an awkwardness of the posture and the breath. If we try to deal with these psychological problems only psychologically, without taking care of the posture and breath, our treatment will be futile. Or it might even make the situation much worse.
In order to regulate the body we must have a keen sensitivity, as I mentioned before. For the sensitivity to function fully, we have to have calmness and clarity, achieved by regulating the mind. Our effort to regulate our breath naturally leads us to correct our posture. ‘ (Polishing A Tile)