Thirty spokes meet at a hub;
Because the hub is empty, it is useful.
Shape clay into a vessel;
Because the vessel is empty, it is useful.
Cut out doors and windows to make a house;
Without these holes, the house couldn't be lived in.
A thing gets profit from what is there;
it gets usefulness from what is not.
Most people think that to add usefulness, you have to add to what is there. This passage emphasizes how the usefulness of things depends also on what is not there. Imagine a life without any free time; although you would be accomplishing a lot, you wouldn't be happy. You need the "empty" time to reflect on and plan for the activity, and to rest from it. The meaning of this passage doesn't just concern the necessity of emptiness, though, but also the proper balance between what is there and what's not. During the school year students often wish they could be free of school forever. During their two-week winter break, they revel in their free time and wish they could have more of it. But during the summer, after a month or two, they become bored and miss the activities they would do during the year. Just like one puts together clay and emptiness in the right proportion to make a useful pot, one needs to put together activity and rest in the right proportion to make a satisfying life.