The thirty spokes unite in the one nave; but it is on the empty
space (for the axle), that the use of the wheel depends. Clay is
fashioned into vessels; but it is on their empty hollowness, that
their use 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
use depends. Therefore, what has a (positive) existence serves for
profitable adaptation, and what has not that for (actual) usefulness.