A vessel is formed from a lump of clay with care, however, it is the empty space within the vessel that makes it useful.
LaoziUnder heaven all can see beauty as beauty only because there is ugliness. All can know good as good only because there is evil. Therefore having and not having arise together. Difficult and easy complement each other. Long and short contrast each other. High and low rest upon each other. Voice and sound harmonize each other. Front and back follow one another.
Laozi