Dogen said [in Gyoji]:
Master Bodhidharma sat in stillness facing the wall, but he was not learning Zen concentration.
and also [in Fukanzazengi]:
Zazen is simply the peaceful and joyful Gate of Dharma.
Stillness is suchness. We fall backwards into it from the discriminating mind. It is always present. The trees are still. The wind is still. It is suchness, not the absence of movement.
At great cost, the ego keeps us suspended several inches above the ground. Zazen is not learning concentration. It is learning to fall.