Usefulness Is From Emptiness

We join spokes together in a wheel, but it is the center hole that makes the wagon move.

The usefulness of the wheel is always in that empty innermost. We shape clay into a pot, but it is the space inside that holds whatever we want. The usefulness of the pot is always in that empty innermost.

We cut walls, floor, ceiling, doors and windows for a house, but it is the inner space that makes it livable. The usefulness of a house is always in that empty innermost.
 
We work with being, but non-being is what we use. 

Profit comes from external form, but usefulness comes from the emptiness

Therefore, something is shaped into what is; but its usefulness comes from what is not.