Clay is fashioned into vessels; it is on their empty hollowness that their use depends. Doors and windows are cut out to make a dwelling, and on the empty space within, its use depends. Thus, while the existence of things may be good, it is the non-existence in them that makes them serviceable.
When the people of the world all know beauty as beauty, There arises the recognition of ugliness. When they all know the good as good, There arises the recognition of evil.