釋禪波羅蜜次第法門 7.57

7.57

Furthermore, because the breath is short, one perceives the mind to be subtle; because the breath is long, one perceives the mind to be coarse. Why is this? As the mind becomes increasingly still, the exhalation is exhausted from the navel to the chest, and the inhalation is known to be exhausted from the nose to the throat. This means that because the mind is still, one perceives the breath to be short. As for perceiving it to be long meaning that the mind is coarse: because the practitioner's mind is coarse, he perceives that from the navel to the nose and from the nose to the navel, the route of the breath is long and distant; this means that because the mind is coarse, one perceives the breath to be long.