Blessed James of Viterbo (c. 1255 – 1308), known as Giacomo da Viterbo, Jacobus de Viterb(i)o, surname Capocci, and nicknamed Doctor speculativus, was an Augustinianfriar and student of Giles of Rome.
He wrote on the relationship between ecclesiastical and temporal power in his book De Regimine Christiano. It argued that although human power alone is lawful, it could only be perfected through the influence of a spiritual power. Walter Ullmann says that it was 'the first systematic exposition of the concept of the Church'.
^Antony Black, Political Thought in Europe 1250-1450 (1992, p.49): [James and Giles, i.e. Aegidius] were members of the order of Eremitical Augustinians, both had studied at Paris (Aegidius probably under Aquinas, James probably under Aegidius), and rose to be archbishops.
^Medieval Foundations of Renaissance Humanism, p.144