I've been trying to remember this all evening...

Because of dangerous levels of intoxication, had too much trouble coherently saying:

11.
Thirty spokes join together in the hub.
It is because of what is not there that the cart is useful.
Clay is formed into a vessel.
It is because of its emptiness that the vessel is useful.
Cut doors and windows to make a room.
It is because of its emptiness that the room is useful.
Therefore, what is present is used for profit.

But it is in absence that there is usefulness.

And now I shall sleep. :)