‘For zazen, we arrange the circumstances in the zendo so that it is not too bright or too dark, not too cold or too hot, not dry or wet. We also arrange the external physical conditions, such as our posture and the amount of food we eat. If we eat too much we may fall asleep pretty easily, so we have to fill just sixty or seventy percent of our stomach. Also, we keep our eyes open, because if we close our eyes we might fall asleep, or we are more likely to enjoy ourselves with lots of imaginings and daydreams. Next we arrange our internal physical condition, that is our heart, our intestines, our stomach and our blood. But these things are beyond our control, so how can we take care of them? The only way is to take care of the breath. If we take care of the breath, very naturally, internal physical conditions will work pretty well. This is important. If we arrange the circumstances around our body, our mind, and all internal and external conditions, then, very naturally, the mind is also engaged in our activities. Then we are not bothered by the workings of our mind; the mind does not touch the core of our existence; it is just with us, that is all. When all circumstances are completely peaceful, just the center of ourselves blooms. This is our zazen; this is shikan taza.’ (Returning to Silence)
This is a passage I have used in teaching before.