Fukanzazengi V: 'Sit Non-thinking'.

Mountains unperturbed by passing clouds.

Next, Master Dogen throws light on the essential, or 'pivotal', matter of zazen:

"When the physical posture is already settled, make one complete exhalation
and sway left and right. Sitting immovably in the mountain-still state, “Think
about this concrete state beyond thinking.” “How can the state beyond thinking
be thought about?” "It is different from thinking" [or "it is non-thinking"]. This is just the pivot of zazen."

He advises that we should first settle into the physical posture. Then we exhale and sway or rock a little from side to side (generally about six or eight times or thereabouts); this is a nice way to stretch and loosen up a bit, and it helps us find a central point of balance for our upper body. Sitting still, upright and strong like a mountain, we 'think beyond thinking' or 'think non-thinking' as it has also been translated.

This may seem like an unusual term. How do we 'think non-thinking'? Well, we don't sit trying to directly 'not think', that would be really frustrating because thoughts would likely just keep coming up frustrating our effort. 'Non-thinking' is not just 'not thinking' because thoughts are naturally arising and present, but in 'non-thinking' we don't grab on to our thoughts and get involved with them, or reject them or try to suppress them (which is just getting involved with 'em in another way). We just let them come and go. 'Non-thinking' is just letting thoughts come and go. If we find ourselves grabbing onto a thought and thinking about something (as we often will) we just stop it and return to sitting letting our thoughts come and go again. That's zazen.

When we're sitting 'non-thinking' thus we can say that we are 'beyond thinking'; we aren't being pulled around by our usual reactions to our thoughts (judging them as 'good' and 'bad' and all that, remember?) and so we can calmly sit and experience our thoughts just coming and going and take a rest from our usual reactive thinking activity.

The line on 'non-thinking/beyond thinking' is in quotation marks as it comes from a conversation between Master Yakusan Igen and a monk; it's taken from a traditional zen story or koan.