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 make it useful.

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

(Chapter 11, Tao Te Ching – Translation by Gia-fu Feng and Jane English)