The Prosaic Earth is a Earth much like ours, in which each and everything in existence is represented by an higher entity (Nobles and/or Imperators). It's considered the major background of Nobilis though others may be used depending on how faithful your campaign is to the Nobilis Lore.
Much like gods, the Council of Four rules over the Prosaic Earth and assure every mortal and noble respects their laws and wills.