A wheel is useful because it has emptiness at its centre,
through which an axle might pass.
A bowl is useful because it is molded around emptiness,
waiting to be filled.
A house is useful because of its doors and windows,
that allow people to enter and live happily.
Therefore the 'what is' is benefited by the 'what is not'.
Each is served by the other.
— Lao Tzu, Tao Te Ching