Though thirty spokes may form the wheel,
it is the hole within the hub
which gives the wheel utility.
It is not the clay the potter throws,
which gives the pot its usefulness,
but the space within the shape,
from which the pot is made.
Without a door, the room cannot be entered,
and without windows it is dark.
Such is the utility of non-existence
Taoism - Stan Rosenthal's Tao Te Ching - Translation