48) and the "Liber Pontificalis," his father was Pope Sergius III (904–911), ("Johannes, natione Romanus ex patre Sergio papa," "Liber Pont." ed. Duchesne, II, 243). Ferdinand Gregorovius,[2] Ernst Dümmler, Thomas Greenwood (Cathedra Petri: A Political History of the great Latin Patriarchate), Philip Schaff, and Rudolf Baxmann[3] agree with Liutprand that Pope Sergius III fathered Pope John XI.[4] If that is true, John XI would be the only known illegitimate son of a Pope to have become Pope himself.