Why do we do zazen staring at a wall? I know that Dogen's instructions for zazen say (or at least intimate) to do this, but does he anywhere give an explanation of why the practice is done like this?

What prompts this question is reading about Buddha having sat underneath a tree, thus presumably outdoors. Things must have been moving in his field of vision. And things are always moving. Even when I'm sitting in front of a wall, my heart is beating, I'm breathing, blood is flowing...things are in constant motion. So why do we practice in such a way as to 'artificially' create a field of vision in which there is no motion?

So: what's up with the wall?

(Sorry if there's a very obvious answer to this that I'm missing; I'm really new at this, please bear with me.)