Brian Tierney, The Crisis of Church and State, 1050–1300 (Toronto, 1988), p. 172; J. H. Shennon, The Origins of the Modern European State 1450–1725 (London, 1974); Quentin Skinner, The Foundations of Modern Political Thought, 2 vols (Cambridge, UK, 1978), 1, p. xxiii; A. Fall, Medieval and Renaissance Origins: Historiographical Debates and Demonstrations (London, 1991), p. 120