Thirty spokes converge upon a single hub;
It is on the hole in the center that the purpose of the axle depends
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 habitable
Thus while the tangible has advantages;
It is the intangible that makes it useful
from Lao Tzu (c. 550 B. C.)