The thirty spokes unite in the one nave; but it is on the space where there is nothing that the utility of the wheel depends. Clay is fashioned into vessels; but it is on their empty hollowness that their utility depends. The door and windows are cut out (from the walls) to form an apartment; but it is on the empty space (within), that its utility depends. Therefore, what has a (positive) existence serves for profitable adaptation, and what has not that for actual usefulness.
Thirty spokes share one hub; it is the hole that makes it useful. Explains how 'Non-being' is the key source of utility.