Legge's nave is the hub, useful because it leaves room for the axle. The chapter may be giving absence a little too much credit: a missing wheel won't take you far. Still, every useful thing in the house is mostly the part that isn't there, the bowl, the cup, the pause in a conversation where the other person finally says it. We are quick to fill a silence, and quicker with the people we love. Today, in one conversation, leave the pause where it falls, a whole breath longer than is comfortable, and find out what it was holding room for.
The thirty spokes unite in the one nave; but it is on the empty space (for the axle), that the use of the wheel depends. Clay is fashioned into vessels; but it is on their empty hollowness, that their use depends. The door and windows are cut out (from the walls) to form an apartment; but it is on the empty space (within), that its use depends. Therefore, what has a (positive) existence serves for profitable adaptation, and what has not that for (actual) usefulness.
Tao Te Ching, ch. 11 (trans. James Legge)