Wu is nothingness, emptiness, non-existence
Thirty spokes of a wheel all join at a common hub,
yet, only the hole at the center allows the wheel to spin.
Clay is molded to form a cup,
yet, only the space within allows the cup to hold water.
Walls are joined to make a room,
yet, only by cutting out a door and a window can one enter the room and live there.
Thus, when a thing has existence alone, it is mere dead-weight.
Only when it has Wu, does it have life.