Because the eye gazes but can catch no glimpse of it, It is called elusive. Because the ear listens but cannot hear it, It is called the rarefied. Because the hand feels for it but cannot find it, It is called the infinitesimal. ... These are called the shapeless shapes, Forms without form, Vague semblances. Go towards them, and you can see no front; Go after them, and you see no rear.