What is Not

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;
But it is on the spaces where there is nothing that the usefulness of the house depends.
Therefore, just as we take advantage of what is, so we should recognise the usefulness of what is not.

Lao Tzu