Thirty spokes converge at the wheel’s hub, to a hole that allows it to turn.
Clay is shaped into a vessel, to enclose an empti­ness that can be filled.
Doors and windows are cut into walls, to provide access to their protection.
Though we can only work with what is there, use comes from what is not there.

—Lao Tsu.

(determined with respect to an essential absence.)
Incomplete Nature; via Dee