For Saul Bellow the essential quest is spiritual: it is a search for humanness in a world that daily assaults and denies such a search. This struggle to be human is the author's one story and the various versions of that same story simply indicate the individual progress each protagonist—Joseph, Asa, Wilhelm, Herzog, Sammler—makes on that journey. To find the genuinely human is the hero's task.
Loris, Michelle, "Mr. Sammler's Planet: The Terms of the Covenant" (1978). English Faculty Publications. Paper 3.