Among the theories prevalent in the middle ages was one that mankind formed a unity, with the pope and the emperor at the head of it: the universal Church and the universal emperor ruled the world (Rehm, Geschichte der Rechtswissenschaft, p. 198.) Even to Leibnitz, writing in the 17th century, it seemed that "totam Christianitatem unam velut Rempublicam componere, in qua Caesari auctoritas aliqua competit" (Opera, 4.330).
Rehm, Allgemeine Staatslehre 18 99); Franklin H.
Rehm, Modernes Fiirstenrecht (Munich, 1904).