We put thirty spokes together and call it a wheel;
But it is on the space where there is nothing that the usefulness of the wheel depends.
We turn clay to make a vessel;
But it is on the space where there is nothing that the usefulness of the vessel depends.
We pierce doors and windows to make a house;
And it is on these spaces where there is nothing that the usefulness of the house depends.
Therefore just as we take advantage of what is, we should recognize the usefulness of what is not.
 From the Tao Te Ching