釋禪波羅蜜次第法門 7.49

7.49

復次，知麁細者，入息則麁，出息則細。何故爾？入氣利急故相麁，出息澁遲故細。復次，知輕重，知入息時輕，出息時重。何以故？入息既在身內，即令體輕；出息時，身無風氣，則覺身重。

Furthermore, regarding knowing the coarse and the subtle: the in-breath is coarse, and the out-breath is subtle. Why is this? Because the incoming air is sharp and swift, its characteristics are coarse; because the out-breath is sluggish and slow, it is subtle. Furthermore, knowing lightness and heaviness: knowing that during the in-breath it is light, and during the out-breath it is heavy. Why is this? When the in-breath is inside the body, it makes the body light; during the out-breath, when the body lacks air, one feels the body is heavy.