The usefullness of emptiness

Thirty spokes converge upon a single hub,
It is the hole in the center that the use of the cart hinges.
We make a vessel from a lump of clay,
It is the empty space within the vessel that makes it useful.
We make doors and windows for a room,
But it is these empty spaces that make the room livable.
Thus, while the tangible has its advantages,
It is the intangible that makes it useful.