In the presence of his legitimate sons from his later marriage, Niccoló needed to prove that Leonello was qualified to be chosen as his successor. Niccoló defended Leonello's rights to succession on three grounds: Leonello's personal attributes, popularity among his subjects, and official papal recognition. Leonello was often praised for exhibiting strong leadership skills and virtuous characteristics, which would make him a worthy heir and future ruler of Ferrara. In addition, Leonello was taught under esteemed humanist teachers such as Guarino da Verona. His father noted that this high level of education further distinguished Leonello from his siblings. Leonello was also popular among the people and his father recognized the wide support that Leonello would receive from his subjects, who approved of Leonello being chosen as Niccoló's successor. Furthermore, the papacy officially recognized Leonello as a legitimate heir. Thus, by 1434, Leonello was given a great deal of authority to co-rule with his father.