The truly great ones rely on substance, and not on surface, hold on to the fruit, and not to the flower.
LaoziClay 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.
Laozi