we should recognize the usefulness of what is not.

 We put thirty spokes together and call it a wheel;

But it is in space where there is nothing that the
usefulness of the wheel depends on.

We turn clay to make a vessel;
But it is in space where there is nothing that the
usefulness of the vessel depends on.

We pierce doors and windows to make a house;
And it is on this space 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.

   - Lao-Tzu (4th century BCE)