Thirty spokes share the wheel's hub;
It is the center hole that makes it useful.

Shape clay into a vessel;
It is the space within that makes it useful.

Cut doors and windows for a room;
It is the holes which makes it useful.

Therefore profit comes from what is there;
Usefulness from what is not there.

- Tao Te Ching



三十輻共一轂 當其無 有車之用

埏埴以爲器 當其無 有器之用

鑿戶牖以爲室 當其無 有室之用

故有之以爲利 無之以爲用

- 道德經