Dune is packed full of of symboligy, but the chapter separations form Princess Irulan's various 'books' set and frame the story and attempt to strongly place the story uniquely within the Dune universe, and world. However, the themes in this novel are much more universal then Irulan presents. She states that "to begin your study of the Muad'Dib, then, take care that you first place him in his time", in accordance to the Bene Gesserit rule for balancing all things at the beginning. Firstly the Bene Gesserit did not do a great job with their planning and both prized the human and yet at the same time wanted complete control.Secondly, although Paul was shaped by Arrakis, his time at Caladan shaped him as well. Herbert here is trying to situate this story as something far away, maybe to allow people to recognize the issues present in a completely alternate world first before relating them to our world here.
Another passage, this time from Muad'Dib's Collected Sayings, (126) speaks of the necessity of maintaining a sufficient amount of skepticism or "even occasional greatness will destroy a man." In this book, the saying seems as it came from Lady Jessica just as much as from Paul since she is the one who constantly comments on how certain mantels should not be taken up, this qualification would provide justification for why she does continue to take on the religious mantel for herself as well as for Paul.