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.
無用, 'The Use of what has no Substantive Existence.' The three illustrations server to set forth the freedom of the Dao from all pre-occupation and purpose, and the use of what seems useless.