Joseph finally finds his way through the metaphors, while Rosen proves himself a master of the more prosaic but rich art of the simile.
Joseph deals with metaphors while Rosen deals with similes. 
mnli hypothesis: Joseph deals with metaphors while Rosen deals with similes.  premise: Joseph finally finds his way through the metaphors, while Rosen proves himself a master of the more prosaic but rich art of the simile.