The function of the non-existent

"Thirty spokes unite in one nave and on that which is non-existent [on the hole in the nave] depends the wheel's utility. Clay is moulded into a vessel and on that which is non-existent [on its hollowness] depends the vessel's utility. By cutting out doors and windows we build a house and on that which is non-existent [on the empty space] depends the house's utility. Therefore, when the existence of things is profitable, it is the non-existent in them which renders them useful."