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.)