Simplicity, what?

Cohen begins the Kepler chapter by saying that "since Greek times scientists have insisted that Nature is simple" (p127). Kepler thought that he was simplifying the cosmological structure by introducing elliptical orbits for the planets, but he could be refuted by someone who insists that circles are more simple than ellipses. Cohen goes on to say that the idea that nature must be simple poses a problem for scientists today as they discover the complexity of the subatomic structures.

First, why must nature be simple? Is this just some quaint, outdated Aristotelian notion, like the 4 elements that all go to their natural places?

Second, if we were to agree that nature must be simple, who determines what simple means? Is an elliptical orbit simpler than a circular orbit if the elliptical orbit means that all the planets orbit around the same point?