Tao Te Ching 11

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.