Thirty spokes converge
In the hub of a wheel;
But the use of the cart
Will depend on the part
Of the hub that is void.

With a wall all around
A clay bowl is molded;
But the use of the bowl
Will depend on the part
Of the bowl that is void.

Cut out windows and doors
In the house that you build;
But the use of the house
Will depend on the part
Of the house that is void.

Thus advantage is had
From whatever is there;
But usefulness arises
From whatever is not.

XI, Lao Tzu.
Tao Te Ching.