We join spokes together to make a wheel, but it is the center hole that makes it useful. We shape clay to make a pot, but it is the emptiness inside that makes it useful. We join wood together to make a house, but it is the inner space, the cut-out doors and windows, that make it livable, and thus useful. We concentrate on being, but it is non-being that gives us life and makes us useful. Having something is beneficial; having nothing is useful. "O" - it's the hole thing that makes all things useful. The usefulness of a being is measured by how much non-being is contained in it.