The essence of things in the Dao

Thirty spokes meet at a single hub. It is precisely because the hub is empty at the center that the cart can do its job. Clay is shaped into a vessel. It is precisely because the vessel is empty inside that it can do its job. Doors and windows are cut out to make a room. It is precisely because the room is empty inside that it can do its job. So what is “there” gives us convenience, while what is “not there” makes things useful.