The Thinker and His Project
Thomas Aquinas (1225-1274), a Dominican friar born to a noble family at Roccasecca in the Kingdom of Naples, produced the most comprehensive synthesis of Christian theology and Aristotelian philosophy in the history of Western thought. His Summa Theologica (begun 1265, left unfinished at his death in 1274) runs to approximately 1.8 million words across three parts and addresses virtually every question in theology, ethics, and metaphysics. The treatise on law (Prima Secundae, Questions 90-97) is the foundational text of natural law theory, the intellectual tradition that undergirds Western constitutional law, human rights jurisprudence, and Catholic social teaching.
Aquinas wrote during the thirteenth-century intellectual revolution occasioned by the recovery of Aristotle's complete works, which had been preserved and transmitted through Arabic translations by Islamic scholars such as Avicenna (Ibn Sina, 980-1037) and Averroes (Ibn Rushd, 1126-1198). The challenge was immense: Aristotle's philosophy was naturalistic, empiricist, and pagan, apparently incompatible with the Augustinian-Neoplatonic tradition that had dominated Christian thought for eight centuries. Aquinas's achievement was to demonstrate that Aristotelian philosophy, properly understood, was not only compatible with Christian revelation but could illuminate and systematize it.
Biblical Texts Engaged
The key text for Aquinas's natural law theory is Romans 2:14-15: 'For when the Gentiles, which have not the law, do by nature the things contained in the law, these, having not the law, are a law unto themselves: which shew the work of the law written in their hearts, their conscience also bearing witness' (KJV). Paul's claim that Gentiles know the moral law 'by nature' - without access to the Torah - establishes that moral knowledge is universally accessible through human reason. This is the scriptural foundation of natural law: the moral law is not arbitrary divine command but is rational, knowable, and grounded in human nature itself.
Psalm 4:6 - 'There be many that say, Who will shew us any good? LORD, lift thou up the light of thy countenance upon us' - provides Aquinas with his epistemology of natural law. The 'light of thy countenance' is the lumen naturale rationis, the natural light of reason, by which human beings apprehend first moral principles. Aquinas interprets the Psalmist as identifying the source of moral knowledge in the divine light imprinted on the human intellect at creation - a participation in God's own eternal reason.
Romans 13:1-2 - 'Let every soul be subject unto the higher powers. For there is no power but of God: the powers that be are ordained of God' - provides the framework for understanding human law's relationship to natural and eternal law. Just human laws derive their binding force from their participation in natural law, which itself participates in God's eternal law. A human law that contradicts natural law is, in Aquinas's famous formulation, 'not a law but a corruption of law' (non lex sed corruptio legis, I-II, Q. 95, Art. 2).
Core Argument
Aquinas's theory distinguishes four types of law arranged in a hierarchy. Eternal law (lex aeterna) is God's rational governance of the entire universe - the divine wisdom directing all things to their proper ends. Natural law (lex naturalis) is the 'participation of the eternal law in the rational creature' (I-II, Q. 91, Art. 2) - the portion of eternal law that human beings can know through reason. Human law (lex humana) is the specific legal enactments derived from natural law principles through the application of practical reason to particular circumstances. Divine law (lex divina) is the law revealed in Scripture, necessary because human reason is fallible, because some truths exceed reason's capacity, and because human beings need certainty about the ultimate purpose of their existence.
The first principle of natural law is synderesis: 'good is to be done and pursued, and evil is to be avoided' (I-II, Q. 94, Art. 2). From this self-evident principle, Aquinas derives three levels of natural law precepts. First-order precepts correspond to the fundamental inclinations of human nature: self-preservation (shared with all substances), procreation and the education of offspring (shared with all animals), and the pursuit of truth and life in society (unique to rational beings). Second-order precepts are conclusions drawn from first principles through 'something like a deduction' - for example, the prohibition of murder derives from the conjunction of the first principle ('good is to be done') and the natural inclination to self-preservation. Third-order precepts require more complex reasoning and prudential judgment and may vary across cultures.
Crucially, Aquinas argues that natural law is accessible to all rational beings, regardless of religious belief. A Buddhist, a Muslim, an atheist, and a Christian can all apprehend the basic precepts of natural law through the exercise of reason. This universality is what makes natural law the foundation of international human rights: it provides a moral vocabulary that transcends particular religious traditions while (in Aquinas's view) ultimately grounding in divine reason.
Intellectual Context
Aquinas was responding to several intellectual challenges. Against the 'Latin Averroists' (particularly Siger of Brabant), who argued that Aristotelian philosophy and Christian theology could reach contradictory conclusions both of which were true, Aquinas insisted on the unity of truth: faith and reason cannot ultimately contradict each other, since both derive from God. Against the Augustinian-Franciscan tradition (particularly Bonaventure), which was suspicious of Aristotle and preferred illumination theory to natural reason, Aquinas argued that grace perfects nature rather than replacing it - that human reason, even in its fallen condition, retains the capacity to know fundamental moral truths.
The Islamic philosophical tradition was essential to Aquinas's project. Avicenna's metaphysics of essence and existence and Averroes's commentaries on Aristotle provided much of the philosophical apparatus that Aquinas Christianized. The Jewish philosopher Maimonides (Moses ben Maimon, 1138-1204), whose Guide for the Perplexed Aquinas read in Latin translation, provided a model for synthesizing religious revelation with Aristotelian philosophy. Aquinas cites 'Rabbi Moses' (Maimonides) hundreds of times in the Summa.
Reception and Critique
Aquinas's natural law theory has been received very differently across Christian traditions. The Catholic Church adopted Thomism as its official philosophical framework. Pope Leo XIII's encyclical Aeterni Patris (1879) declared Aquinas the preeminent Catholic philosopher. The Second Vatican Council's Declaration on Religious Freedom (Dignitatis Humanae, 1965) drew on Thomistic natural law to ground religious liberty in human dignity. Catholic social teaching - from Rerum Novarum (1891) through Laudato Si' (2015) - consistently applies natural law reasoning to social, economic, and environmental questions.
The Protestant Reformers were more critical. Luther dismissed natural law as compromised by sin, arguing that fallen reason cannot reliably know God's moral will apart from Scripture. Calvin was somewhat more sympathetic, acknowledging a 'sense of divinity' (sensus divinitatis) implanted in all humans, but he rejected the Thomistic confidence in reason's moral capacity. Karl Barth mounted the sharpest Protestant critique, arguing that natural theology of any kind - including natural law - represents a human attempt to know God apart from Christ and must be rejected as idolatry.
Secular critics have attacked natural law from multiple angles. David Hume's 'is-ought' problem (Treatise of Human Nature, 1739) - the argument that moral conclusions cannot be derived from factual premises - challenges Aquinas's attempt to derive moral norms from human nature. G.E. Moore's 'naturalistic fallacy' (Principia Ethica, 1903) raises a similar objection. Legal positivists from John Austin to H.L.A. Hart argue that law is a social fact, not a moral category, and that Aquinas's conflation of law and morality confuses the analysis.
Defenders respond that Aquinas is not committing the naturalistic fallacy because his metaphysics is teleological: human nature is directed toward certain ends (telos) by God, and these ends define what is good for humans. The 'is' from which Aquinas derives 'ought' is not a bare fact but a nature imbued with purpose. The 'new natural law' theorists - John Finnis (Natural Law and Natural Rights, 1980), Germain Grisez, and Robert George - have developed sophisticated defenses of Thomistic natural law against modern objections.
Legacy and Influence
Aquinas's natural law theory is one of the most consequential intellectual contributions in Western history. It undergirds the entire tradition of human rights, from the Spanish scholastics (Vitoria, Suarez) through Grotius and Locke to the Universal Declaration of Human Rights. Jacques Maritain, the principal philosophical influence on the UDHR, was an explicit Thomist. The concept of 'inalienable rights' - rights that cannot be surrendered or removed because they are grounded in human nature rather than positive law - is a Thomistic concept.
In legal philosophy, Aquinas's dictum that 'an unjust law is not a law' has been invoked in landmark moments. Martin Luther King Jr. cited Aquinas directly in his Letter from Birmingham Jail (1963): 'An unjust law is a human law that is not rooted in eternal law and natural law. Any law that uplifts human personality is just. Any law that degrades human personality is unjust.' The Nuremberg Trials (1945-1946) implicitly relied on natural law reasoning to prosecute Nazi war criminals for acts that were legal under German positive law.
Key Passages
From the Summa Theologica, I-II, Q. 91, Art. 2: 'Since all things subject to Divine providence are ruled and measured by the eternal law... it is evident that all things partake somewhat of the eternal law, in so far as, namely, from its being imprinted on them, they derive their respective inclinations to their proper acts and ends. Now among all others, the rational creature is subject to Divine providence in the most excellent way, in so far as it partakes of a share of providence, by being provident both for itself and for others. Wherefore it has a share of the Eternal Reason, whereby it has a natural inclination to its proper act and end: and this participation of the eternal law in the rational creature is called the natural law.'
From I-II, Q. 95, Art. 2: 'Every human law has just so much of the nature of law, as it is derived from the law of nature. But if in any point it deflects from the law of nature, it is no longer a law but a perversion of law.'
From I-II, Q. 94, Art. 2: 'The first principle of practical reason is one founded on the notion of good, namely, that good is that which all things seek after. Hence this is the first precept of law, that good is to be done and pursued, and evil is to be avoided. All other precepts of the natural law are based upon this.'
Contemporary Relevance
Natural law theory remains at the center of contemporary debates in legal philosophy, bioethics, and political theory. John Finnis's Natural Law and Natural Rights (1980) revived Thomistic natural law for the analytical philosophy tradition. Alasdair MacIntyre's After Virtue (1981) and Whose Justice? Which Rationality? (1988) argue that the Thomistic tradition offers the most coherent moral framework available in a fragmented intellectual world. The natural law tradition is central to Catholic opposition to abortion, euthanasia, and the redefinition of marriage, as well as to Catholic advocacy for workers' rights, environmental stewardship, and economic justice.
The dialogue between natural law and contemporary secular philosophy continues. Jurgen Habermas, in his 2004 dialogue with Joseph Ratzinger, acknowledged that secular reason may need to draw on religious traditions - including natural law - to sustain the moral commitments that liberal democracy requires. The question of whether human rights can be grounded without recourse to natural law, and whether natural law can be articulated without recourse to theology, remains one of the most important intellectual questions of our time.