‘When we are sitting, we do not follow our thoughts, nor do we stop them. We just let them come and go freely. We cannot call it thinking because the thoughts are not grasped. If we simply peruse our thoughts, it is just thinking; it is not zazen. We cannot call zazen not-thinking either, because thoughts are coming and going like clouds floating in the sky. When we are sitting, our brain does not stop functioning, just as our stomach is always digesting. Sometimes our minds are busy; sometimes our minds are calm. Just sitting, without being concerned with the conditions of our mind, is the most important point in zazen. When we sit in this way, we are one with Reality, which is beyond thinking. To say it another way, Reality manifests itself through our body and mind.’ (notes on Fukanzazengi)
When I was trying to find where I had mentioned “think of not thinking” before, for yesterday’s post, I came across this one, which also helps.