Lao Tzu: "The Uses of Not"

THE USES OF NOT

Thirty spokes
meet in the hub.
Where the wheel isn't
is where it's useful.

Hollowed out,
clay makes a pot.
Where the pot's not
is where it's useful.

Cut doors and windows
to make a room.
Where the room isn't,
there's room for you.

So the profit in what is
is in the use of what isn't.

--Lao Tzu

from Ursula K. Le Guin's
Lao Tzu: Tao Te Ching: 
A Book About the Way and the Power of the Way