Thirtyspokessharethewheel'shub;Itisthecenterholethatmakesituseful.
Shapeclayintoavessel;Itisthespacewithinthatmakesituseful.
Cutdoorsandwindowsforaroom;Itistheholeswhichmakeituseful.
Thereforebenefitcomesfromwhatisthere;Usefulnessfromwhatisnotthere.