The use of non-existence

"Thirty spokes unite in one nave, and by that part which is non-existent (i.e. the hole in the centre of it) it is useful for a carriage wheel. Earth is moulded into vessels, and by their hollowness they are useful as vessels. Doors and windows are cut out in order to make a house, and by its hollowness it is useful as a house. So then existence may be said to correspond to gain, but non-existence to use."