The Sixth Patriarch heard two monks arguing about the temple flag waving in the wind.
One said “It’s the flag which moves”
The other said “It’s the wind which moves”
The Sixth Patriarch said “It’s the mind which moves”
Commentary: The Sixth Patriarch isn’t saying that the mind moves, but the flag and the wind are still. He doesn’t say that the mind shouldn’t move. He doesn’t say that if the mind was still, then everything would be still.
Everything: flag, wind, mind is dependent origination. To imagine that the mind is separate, is “mine” and can and should be quietened is the root of suffering.
Because everything is dependent origination, everything moves and everything is still.
Zazen is the vast space which holds everything.