The Principle
Francisco Suárez (1548-1617) is considered alongside Hugo Grotius as a founder of modern international law. A Jesuit philosopher and theologian of extraordinary precision and range, Suárez developed in De Legibus ac Deo Legislatore (1612) the most systematic Catholic theology of law produced in the early modern period. His treatise distinguished natural law, divine positive law (Scripture), and human positive law in ways that became foundational for Catholic legal philosophy and provided the conceptual architecture for modern international humanitarian law. Where Grotius would later argue that natural law bound nations even without God, Suárez insisted that the theological grounding of law was not negotiable - an argument whose implications continue to be debated in legal philosophy.
Biblical Foundation
Suárez grounded his theology of law in three Pauline texts. Romans 2:14 - "the Gentiles, which have not the law, do by nature the things contained in the law" - established natural law's universal accessibility. Romans 13:1 - "Let every soul be subject unto the higher powers. For there is no power but of God" - provided the theological legitimation of civil authority. Proverbs 8:15 - "By me kings reign, and princes decree justice" - grounded the claim that divine wisdom (which Suárez identified with eternal law) was the ultimate source of all legitimate authority. Suárez also engaged the Sermon on the Mount extensively in his ethics, treating Matthew 5-7 as the normative standard against which positive law must be measured. His discussion of just war drew on Deuteronomy 20 and Augustine's canonical treatment, while his theory of ius gentium (the law of nations) drew on Acts 17:26 to establish the natural unity of all peoples under a common rational law.
Historical Transmission
Suárez's De Legibus was immediately influential. Grotius read Suárez carefully and engaged his arguments throughout De Jure Belli ac Pacis (1625), accepting the distinction between natural law and ius gentium while rejecting Suárez's insistence on a voluntarist God as natural law's foundation. The Jesuit missions to Asia and the Americas used Suárez's framework to develop mission law and engage questions of indigenous rights, creating a direct practical application of his theoretical work. Suárez's treatise on war in De Triplici Virtute Theologica (1621) developed just war theory in ways that influenced Grotius's systematic formulation and, through Grotius, the entire subsequent tradition of international humanitarian law. His distinction between natural law (immutable, known by reason), divine positive law (Scripture, binding Christians), and human positive law (varying by community) became the standard Catholic framework for analysing the relationship between religious and secular authority.
Modern Application
Suárez's influence on modern international law operates primarily through Grotius, who transmitted a rationalised version of the Suárezian framework to the Protestant legal tradition. The Hague Conventions (1899, 1907) and Geneva Conventions (1929, 1949) embody principles traceable to the Salamanca School - Vitoria, Soto, and Suárez - through this transmission chain. The Catholic Church's continued engagement with international law, from Leo XIII's Rerum Novarum (1891) to Vatican II's Gaudium et Spes (1965) to John Paul II's addresses to the United Nations, employs the Suárezian framework of natural law grounded in Scripture and accessible to all through reason. Contemporary Catholic social teaching - including its positions on the just war, refugee protection, and human rights - draws explicitly on Suárez's architecture.
Scholarly Debate
Scholars disagree about whether Suárez is better understood as a continuator of the medieval scholastic tradition or as a transitional figure toward modernity. Quentin Skinner's The Foundations of Modern Political Thought places Suárez in the Conciliarist tradition developing toward popular sovereignty. Francis Oakley's Natural Law, Laws of Nature, Natural Rights argues for strong continuity between Suárez and the medieval voluntarist tradition, contesting the narrative of progressive secularisation. For legal historians, the most important debate concerns whether Suárez's insistence on the theological grounding of natural law makes his framework inapplicable in a pluralist international order - a debate that bears directly on the role of religious traditions in contemporary international law. The answer may be that Suárez's method, which sought to articulate universal norms accessible to all rational persons while grounding them in biblical revelation, anticipated the contemporary challenge of maintaining universal human rights in a religiously diverse world.