Usefulness in nothing

Thirty spokes converge on a single hub,
  but it is in the space where there is nothing
    that the usefulness of the cart lies.
Clay is molded to make a pot,
  but it is in the space where there is nothing
    that the usefulness of the clay pot lies.
Cut out doors and windows to make a room,
  but it is in the spaces where there is nothing
    that the usefulness of the room lies.
Therefore,
  Benefit may be derived from something,
    but it is in nothing that we find usefulness.
~Lao Tzu Tao Te Ching