Walking in physical space expands the mind.

Thoreau once observed how shades made by clouds interacted with his emotional state during a walk:

By the mood of my mind, I suddenly felt dissuaded from continuing my walk, but I observed at the same instant that the shadow of a cloud was passing over the spot on which I stood, though it was of small extent, which if it had no connection with my mood, at any rate suggested how transient and little to be regarded that mood was. I kept on, and in a moment the sun shone on my walk within and without.

Journal, July 23, 1851