Natural Law in Medieval Christian Jurisprudence

By Professor Augusto Zimmermann PhD*

Reason is a natural faculty of the human mind. It confers the ability to understand and to form moral judgments, thus enabling us to distinguish right from wrong, justice from injustice. As for natural law theory, it connects legal issues with timeless principles that are inherent in human nature, ‘not with the laws of any particular nation but with a universal law applicable in all nations’.^[1]

While it is true that a pagan or humanistic version of natural-law theory was touched upon by ancient Greek and Roman scholars, advancement of the concept into Western civilization is primarily due to Christian moral theory emanating from ancient Mosaic precepts, particularly the Ten Commandments. Thomas Aquinas, for example, saw the Decalogue as ‘the epitome of the natural law, broadly understood’.[2]

Natural-law theory underpins the West’s rich legal tradition. However, Christian legal philosopher J. Budziszewski makes the important observation that natural law loses its ‘normative force’ if it is not considered in association with the idea of a higher law that is instituted by a personal Creator.^[3] According to Budziszewski, an explanation of natural law cannot be satisfactorily evaluated based on humanistic considerations regarding whether the distinctive features of natural law are captured by that explanation. In other words, such considerations regarding God’s role as essential explainer of natural moral laws cannot be anything short of fundamental.[4]

If this jurisprudential approach is accepted, then the investigation of ‘divine revelation’ is a valuable and beneficial intellectual exercise. Although it is no longer a matter often addressed by contemporary natural law theorists (who typically avoid discussing complex theological issues), natural law theory is solidly grounded not only in the study of nature but also in the Word of God itself.^[5] Accordingly, natural law expresses God’s general revelation of both physical and moral laws through our conscience, our inherent sense of right and wrong. The Apostle Paul says, ‘Indeed, when Gentiles, who do not have the [Mosaic] law, do by nature things required by the law, they are a law for themselves, even though they do not have the law.’ (Romans 2:14).

This theological assumption obviously does not mean that every argument supporting the natural law (and natural rights) must be accompanied by scriptural confirmation. It does imply, however, that when someone presents an argument aiming at tracing the concept of natural law right back to its more solid philosophical foundations, these foundations can be found in God’s Word, as revealed in Scripture.

The main objective of this short article is to introduce the reader to the general view of natural law according to medieval Christian jurisprudence. Natural law, under the medieval jurisprudential understanding, can be defined in terms of universally applicable laws as revealed in Scripture. Accordingly, medieval scholars believed that law is not ultimately a product of human authority, but derived instead from objective principles that ‘operate whether or not they are recognized or enforced by any government.’^[6] As noted by law professor Jonathan Burnside,

the Bible has important things to say about natural law that squarely address and inform the Western legal tradition. … Augustine, Ambrose, Origen and Jerome all find continuity between divine law (whilst drawing on different parts of Scripture) and natural law.[7] 

The Old Testament

The Old Testament depicts God as both the creator and lawgiver, whose commands to the patriarchs (Adam, Noah, Abraham and Moses) embody and reflect the eternal standards of justice and morality to which every person and society ought to conform. Hence, the early Christian theologian, Irenaeus (c AD 130–200), argued that the Ten Commandments are intrinsic in the natural order.

These commandments are a reminder of the eternal principles that God established since the time of creation.[8] They represent the continuity between the Creator and His creation, with the biblical account revealing basic principles of the natural law. In this perspective, Professor Burnside comments:

Certain moral norms seem to be implicit in the Creation narratives which … God expects all human beings to know and respect. In this sense, there is nothing new about the Decalogue. Its commands and prohibitions do not appear ‘out of the blue’. On the contrary, there is a sense in which they have always been commands and prohibitions. Something can be commanded, not done in creating the universe. By the same token, something can be forbidden, not because God has given a specific commandment, but because it is not consistent with what God has done in creation.[9]

The kings of Ancient Israel were expected to administer the law according to objective standards.[10] Hebrew prophets often drew attention to the injustices perpetrated by kings both within Israel and among the surrounding nations and empires. They called on them to repent and warned them of impending divine judgement.[11] Although Israel amounted at most to a regional power, this tradition acquired a wider influence through the growth and expansion of Christianity in later centuries.

The New Testament

Christian theology advocates that natural laws are revealed through reason and conscience. This is a message biblically conveyed in 1 Corinthians 11:7 and 2 Corinthians 4:4. While nobody can be saved only by knowing of the law, humanity is made ‘in the image of God’ (Genesis 1:26-27) and that the natural law is written in our hearts, implying that nobody has an excuse to disobey this law.

The Gospel of Matthew tells of when Christ was questioned by the Pharisees as to whether it was lawful to pay taxes to Caesar. Christ responded: ‘Render unto Caesar the things that are Caesar’s, and to God the things that are God’s’.[12] There Christ appears to have drawn a distinction between temporal laws and the eternal laws of God. Should the two be in conflict, the laws of God must prevail.[13]

Further evidence is found in Paul’s Letter to the Romans. There Paul says that, although the Gentiles did not directly receive the Ten Commandments, they still can do all things required by them, ‘because of the work of the law that is written in their hearts. Their conscience bear witness of this fact, with their thoughts accusing or else excusing them’.[14] The late British theologian, John Stott, commented:

The same moral law, which God has revealed in Scripture, he has also stamped (even if not so legibly) on human nature. Since he has in fact written his law twice, internally as well as externally, it is not to be regarded as an alien system, which we impose on people arbitrarily, and which it is altogether unnatural to expect human beings to obey. On the contrary, there is a fundamental correspondence between the law in Scripture and the law in human nature. God’s law fits us; it is the law of our own being. We are authentically human only when we obey it. When we disobey it, we are not only rebelling against God, we also contradict our true selves.[15]

Christian jurists have throughout the ages relied on these biblical passages to claim the supremacy of the natural law.[16] Accordingly, laws which are inconsistent with the natural law are not valid, at least not valid in the fullest sense of the word. The early Christian theologian, Origen (c AD 185–254), stated:

Where the law of nature, that is of God, enjoins precepts contradictory to the written laws, consider whether reason does not compel a man to dismiss the written code and the intention of the lawgivers, and to devote himself to the divine Lawgiver and to choose to live according to His word, even if in doing this he must endure dangers and countless troubles and death and shame.[17]

Another early Christian theologian, St Ambrose (c AD 337–397), believed that ‘a conscientious man must sooner die than obey a command which he knows is wrong’.[18] He was able to put this principle into practice. When appointed Bishop of Milan, Ambrose compelled the Roman Emperor, Theodosius, to repent of his vindictive massacre of 7,000 people, in AD 390. The fact is historically relevant. It indicates that, under Christian influence, even the Roman Emperor was expected to obey the law. The institutional office of the absolute king inherited from the Roman law was transformed into a monarch under the law.[19]

Saint Augustine of Hippo (AD 354–430)

Saint Augustine of Hippo, one of the most influential philosophers in the history of the Western world, insisted that the natural law is the foundation of every true law.[20]  Augustine argued that “true law” flows from eternal standards which are embedded in the natural law. This superior law, the natural law, St Augustine declared in The City of God,

is not just a mere cultural invention of Greek and Roman philosophers but it represents the discoveries of men, who, at the prompting of their own speculative ability, made efforts to discover the hidden laws of nature, and the right and wrong in ethics, and in dialectic what was inconsequent and erroneous. And some of them, by God’s help, made great discoveries; but when left to themselves they were betrayed by human infirmity, and fell into mistakes. And this was ordered by Divine Providence, that their pride might be restrained, and that by their example it might be pointed out that it is humility which has access to the highest region.[21]

Augustine’s City of God is widely acclaimed as a classic of Christian theology. There he draws a distinction between the natural law, which is eternal and immutable, and the laws of society that can be applied to different social contexts. Augustine thus contended that the Old Testament instructs us on the reality of laws reflecting God’s wisdom. As Greg Forster points out, ‘the premise Augustine develops here, that the laws of a society represent the application of a universal moral law to the particular circumstances of that society, became one of the crucial building blocks of natural-law theory”.^[22]

St Augustine also contended that social life is by its very nature designed to work in an ordered manner. Hence, even those who reject God in their hearts generally do not reject the natural orderliness of society. ‘The earthly city, whose life is not based on faith, aims at an earthly peace’, or ‘a kind of compromise between human wills about the things relevant to mortal life’.^[23] Whereas sinful people cannot achieve spiritual peace, they still can be ‘at peace with the law by which the natural order is governed’.^[24] Thus, preserving the lawfulness of human society is regarded as a common area of socio-political endeavor, whereas, on a more personal level, people might submit to God or not. ^[25]

Augustine’s view of the natural law was heavily motivated by two fundamental principles: unity, which is derived from God and involving one true Faith, one Church, and one Empire; and the supremacy of the law not merely man-made but instituted as part of the unity of the universe.[26] This duality leads him to conclude that a ‘law that is unjust is not seen to be law at all’.[27] Devoid of justice, the distinction between civil government and a gang of criminals disappears, and a government is no better than a system of organized banditry. As St Augustine put it: ‘Justice being taken away, then, what are kingdoms but great robberies? For what are robberies themselves, but little kingdoms?’[28]

St Thomas Aquinas (AD 1225–1274)

The Dominican friar, St Thomas Aquinas, developed an intense interest in Aristotle’s philosophy. Aquinas’s legal philosophy is now generally regarded as ‘the most perfect conciliation of faith and reason’.^[29] Pope John XII declared Aquinas a saint on 18 July 1323.[30] His legal philosophy is now the official doctrine of the Church on everything pertaining to the natural law.[31] In his Encyclical Aeterni Patris (1879) Pope Leo XIII commends Aquinas for having provided ‘the surest intellectual foundation for, and articulation of, Catholic doctrine’.[32] He also declared Thomism, or the teachings of St Thomas Aquinas, ‘the orthodox system of thought for Roman Catholicism’.^[33]

Aquinas’s Summa Theologica places legality within a broader context of God’s overall design for humanity.[34] As such, human law is defined as ‘an ordinance of reason for the common good, made by him who has care of the community, and promulgated’.[35] And yet, law that cannot be justified by reason is a defective law that ceases to have legitimacy. Nobody needs to obey it unless such disobedience risks undermining a legal system that, overall, might still favor the common good. ‘Wherefore if a case arises wherein the observance of that law would be hurtful to the general welfare, it should not be observed’, Aquinas wrote.[36]

St Thomas Aquinas believed that, as rational creatures, we participate in God’s eternal law ‘in a most excellent way’. This participation is based on compliance with the natural law that is discoverable by reason.[37] As mentioned by him, reason is a ‘God-given natural faculty of the human mind, a capacity to understand and to form judgements’.[38] Accordingly, reason is contrasted to human will, or the faculty to manifest passions and emotions. Reason is above passions and emotions because it is the former that enables us to distinguish what is right from what is wrong. Further, reason allows us to identify law with timeless principles that are inherent in human nature — ‘not with the laws of any particular nation but with a universal law applicable in all nations’.[39] 

Aquinas also considered that the precepts of natural law are apprehended as ‘practical reason’. The first principle of natural law, according to him, is that good is to be done and pursued, and evil is to be avoided. All other precepts are based on this. Whatever reason one might apprehend, this belongs to elementary precepts of the natural law and cannot be avoided because these precepts are unchangeable.[40] The validity of law thereby depends on respecting supremacy of the natural law, so that ‘which is not just seems to be no law at all’. And since ‘the force of a law depends on the extent of its justice, in human affairs’, Aquinas concluded,

a thing is said to be just, from being right, according to the rule of reason. But the first rule of reason is the law of nature … Consequently, 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 the law.[41]

In this sense, the primary purpose of Aquinas’s natural-law theory is to integrate human law with the divine law. Natural law is basically that part of the divine law which is discoverable through reason. Ultimately, however, due to our own intellectual limitations and sinful predispositions he makes it patently clear that knowledge of the law should always be guided by Divine Revelation. As Aquinas points out, 

Human reason is very deficient in things concerning God. A sign of this is that philosophers in their research, by natural investigation, into human affairs, have fallen into many errors, and have disagreed among themselves. And consequently, in order that men might have knowledge of God, free of doubt and uncertainty, it was necessary for divine matters to be delivered to them by way of faith, being told to them, as it were, by God Himself Who cannot lie.[42]

Aquinas’s natural-law theory led to a view of limited government. The principles he supported – the supremacy of the legislature over the judiciary, the independence of the judiciary from political pressure or interference, and the reliance of the judiciary upon the law — are the fundamentals of justice according to the law. For Aquinas, therefore, we are bound to obey government in so far as this is required by the order of justice. ‘Wherefore if the prince’s authority is not just but usurped, or if he commands what is unjust, his subjects are not bound to obey him, except perhaps accidentally in order to avoid scandal or danger’, he stated.[43] As noted by the late Catholic legal philosopher, Charles Rice,

Aquinas analysis is a prescription for limited government, providing a rational basis on which to affirm that there are limits to what the state can rightly do. His insistence that the power of the human law be limited implies a “right” of the person not to be subjected to an unjust law.[44]

Due to the supremacy of the natural law in Thomist legal philosophy, ‘once the king is established, the government of the kingdom must be so arranged that opportunity to tyrannize be removed. At the same time his power should be so tempered that he cannot easily fall into tyranny’.[45] Although this remedy is only available in extreme cases, still Aquinas boldly proclaimed:

If it is a people’s right to provide itself with a king, and if that king tyrannically abuses the royal power, there is no injustice if the community deposes or checks him whom they have raised to the kingship, nor can it be charged with a breach of faith for abandoning a tyrant, even if the people had previously bound themselves to him in perpetuity; because, by not faithfully conducting himself in government as the royal office demands, he has brought it on himself if his subjects renounce their bargain with him.[46]

Other Medieval Christian Thinkers of the Natural Law

The idea of natural law was developed in the writings of all the medieval canonists.[47] If we accept the identity between human law and natural law as universal norms of right behavior, the ideal of legality known as the rule of law is indeed a product of Western political and religious developments, in particular values arising from the Roman Catholic Church of the Middle Ages.[48] 

Original to those medieval canonists was the belief in inalienable rights grounded in principles of the natural law. Hence, Master Rufinus of Bologna, a canonist from the mid-twelfth century,[49] associated natural law with ‘a certain force instilled in every human creature by nature to do good and avoid the opposite’.[50] Likewise, Master Huguccio of Pisa, another Italian canonist of that same century, defined natural law as ‘the judgement of natural reason’ acting to restrain the ‘subjective force or power inherent in human personality’.[51] As noted by the late Harvard legal historian, the celebrated Harold Berman, generally speaking,

The jurists of that time taught that human law, including customary law, derives its legitimacy from natural law, which is in turn, they said, a reflection of divine law. Natural Law was thought to be immediately accessible to human reason. Divine law was revealed to human reason in sacred texts and in the traditions of the church. At the same time, they recognized that human selfishness, pride, and the drive for power are sources of unjust laws, which are contrary both to natural law and to divine law. Thus human law, through a response to divine will, was seen to be also a product of a defective human will which could be, and needed to be, corrected by human reason. Human reason, it was said, coincided with natural law and divine law in postulating that crimes should be punished, that contracts should be enforced, that relationships of trust and confidence should be protected, that accused persons should be heard in their own defense, and, in sum, that legal rules and procedures should conform to standards of justice.[52]

Final Remarks

Medieval Christian thinkers believed in the natural ability of humans to form communities mirrored in rational precepts of the natural law. They embraced natural law as universal norms of right conduct to be respected and applied by everyone and in every circumstance.

In sum, the natural law, as exposed by medieval thinkers, reflects the two sides of God’s nature, namely, His rational judgement and His will prescribing appropriate human behavior. Inspired by these ideas, Christians have over the centuries been prepared to stand up against political tyranny by using natural law thinking as their main weapon – as a form of higher law jurisprudence.

Whatever one makes of these arguments, it is to be noted that biblical principles of the natural law are enshrined in some of the most important documents in Western legal-constitutional history, including the Magna Carta of 1215, the English Bill of Rights of 1689, and the American Declaration of Independence of 1776.[53]

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*      Professor Augusto Zimmermann PhD, LLM (summa cum laude), LLB (Hon.), LLM, CIArb, DipEd is Foundation Dean and Professor of Law at Alphacrucis University College. He also serves as Distinguished Scholar at the Wagner Faith & Freedom Center (Spring Arbor University, Michigan, U.S.) and as a Mentor at Academia Libera Mentis (Theux, Belgium). He is a former Law Reform Commissioner with the Law Reform Commission of Western Australia (2012-2017), and a former Chairperson of the Research Committee and co-Chairperson of the Academic Council at Sheridan Institute of Higher Education, Perth. Professor Zimmermann is also a former Associate Dean (Research) and Post-graduate Research Director at Murdoch University’s School of Law. He is the Founder and President of the Australian Legal Theory Association (ALTA) (formerly The Western Australian Legal Theory Association – WALTA), a former Vice-President of the Australasian Society of Legal Philosophy (ASLP), an Elected Fellow at the International Academy for the Study of the Jurisprudence of the Family (IASJF), and Editor-in-Chief of The Australian Jurist law journal (formerly The Western Australian Jurist). Professor Zimmermann was awarded the 2012 Vice Chancellor’s Award for Excellence in Research, and two School Dean’s Research Awards at Murdoch University, in 2010 and 2011. He served on numerous academic bodies at Murdoch, including: the Research Degree and Scholarships Committee; the Vice Chancellor’s Awards and Citations Committee; the Academic Council’s Freedom of Speech in Policies and Procedures Advisory Group; and the Academic Staff Promotions Advisory Committee. In January 2015, Professor Zimmermann was invited by the Tasmanian Chief Justice to address the ‘Opening of the Legal Year’ in that State. He is generally recognised as a fierce advocate for freedom of speech and the Rule of Law, contributing with numerous articles on the subject, including for The Legal Doctrines of the Rule of Law and the Legal State (Springer, 2014), a book edited by the President of the American Bar Association (ABA). He is the author/co-author/editor/co-editor of numerous academic articles and academic books, including The Legal Right to Disobey Law: A Natural Law Approach to Free Speech and Civil Disobedience, Sidestream Press, 2026; Against Tyranny, Connor Court Publishing 2025; Foundations of the Australian Legal System: History, Theory and Practice, LexisNexis, 2023; Wokeshevism: Critical Theories and the Tyrant Left, Connor Court Publishing 2023; Fundamental Rights in the Age of Covid-19, Connor Court Publishing, 2021; No Offence Intended: Why 18C is Wrong, Connor Court Publishing, 2016; Christian Foundations of the Common Law (3 Volumes), Connor Court Publishing, 2018; Global Perspectives on Subsidiarity, Springer, 2014; and Western Legal Theory: History, Concepts and Perspectives, LexisNexis, 2013. Professor Zimmermann has been included, together with only twelve other Australian academics and policy experts, in ‘Policy Experts’ – the Washington/DC-based Heritage Foundation’s directory for locating knowledgeable authorities and leading policy institutes actively involved in a broad range of public policy issues, both in the United States and worldwide.

[1] Harold Berman, Law and Revolution II: The Impact of the Protestant Reformation on the Western Legal Tradition (Harvard University Press, 2003) p 243.

[2] Thomas Aquinas, Treatise on Law (Hackett, 2000) q98 a6 o3.

[3] J. Budziszewski, Written on the Heart (IVP Academic, 1997) p 210.

[4] See Mark C. Murphy, God and Moral Law: On the Theistic Explanation of Morality (Oxford University Press, 2011) p 69.

[5] Budziszewski, above note 3, pp 183-4.

[6] Randy E. Barnett, ‘A Law Professor’s Guide to Natural Law and Natural Rights’ (1997) 20 Harvard Journal of Law and Public Policy 655, 656.

[7] Jonathan Burnside, Natural Law and Biblical Law, in: Jonathan Crowe and Constance Youngwon Lee, Research Handbook on Natural Law Theory (Chelteham: Edward Elger Publishing, 2019), p 196.

[8] Ireneaus, Adversus omnes haereses 4.13–15.

[9] Jonathan Burnside, God, Justice and Society: Aspects of Law and Legality in the Bible (Oxford: Oxford University Press, 2011), p 69.

[10] Deuteronomy 17:18–20.

[11] For example, II Samuel 12:1–9; Isaiah 1:21–27; Jeremiah 22:1–17; Nahum 3.

[12] Matthew 22:21

[13] See Luke D D Barbrick, ‘The Obedient Rebel: Should Christians Always Obey Their Government?’, in: Augusto Zimmermann and Joshua Forrester (eds.), Against Tyranny (Brisbane/Qld: Connor Court Publishing, 2025), pp 10-11. 

[14] Romans 2:15.

[15] John Stott, The Message of Romans: God’s Good News for the World (Nottingham: Inter-Varsity Press, 1994), p 89.

[16] See Augusto Zimmermann and Gabriël A. Moens AM, The Legal Right to Disobey Law: A Natural Law Approach to Free Speech and Civil Disobedience (Sydney/NSW: Sidestream Press, 2026), pp 49-74.

[17] Origen, Contra Celsum, Bk 5, para 37.

[18] Ambrose, Sermon Contra Auxent, paras 1–2.

[19] Brian Z Tamanaha, On The Rule of Law: History, Politics, Theory (Cambridge: Cambridge University Press, Cambridge, 2004), p 23.

[20] See A H Chroust, ‘The Fundamental Ideas in St Augustine’s Philosophy of Law’ (1973) 18 American Journal of Jurisprudence 57-81.

[21] St Augustine, The City of God, Bk II, Pt VII.

[22] Greg Forster, The Contested Public Square (Downers Grove/IL: Inter-Varsity Press, 2008), p.89. 

[23] Augustine, above note 21, 19.17.

[24] Ibid., 19.13.

[25] Greg Forster, The Contested Public Square (Downers Grove/IL: Inter-Varsity Press, 2008), p.90.

[26] M D A Freeman, Introduction to Jurisprudence (8th ed, Sydney/NSW: Thomson Reuters, 2008), p 99.

[27] Augustine, above note 21, Bk I, Pt V.

[28] Ibid., Bk IV, Pt 4

[29] John Haldane, Faithful Reason: Essays Catholic and Philosophical (London/UK: Routledge, 2004), p 8

[30] John Paul Meenan, ‘Thomas Aquinas: The Common and Universal Doctor’, Catholic Insight, 28 January 2025, at https://catholicinsight.com/2025/01/28/the-enduring-value-of-thomas-aquinas/

[31] Ibid.

[32] Haldane, about n.29, p 8.

[33] Ibid.

[34] Aquinas, Summa Theologica, I, II, Q 93, art 3.

[35] Ibid, I, II, Q 90, art 4.

[36] Ibid, I, II, Q 96, art 6.

[37] Ibid, Q 91, art 2.

[38] Berman, above note 1, p 242.

[39] Ibid, p 243.

[40] Aquinas, above note 34, I, II, Q 94, art 5.

[41] Ibid, Q 95.

[42] Ibid., II, II, Q 2, art 4.

[43] Ibid., II, II, Q 104, art 6.

[44] Charles Rice, 50 Questions on the Natural Law: What It Is & Why We Need It (San Francisco/CA: Ignatius Press 1999), p 85.

[45] Aquinas, De Regimine Principum , Bk I, Ch 2, p 41.

[46] Ibid., Bk 1, Ch 6.

[47] Canon law is the body of laws and regulations made or adopted by ecclesiastical authority for the organisation of the Church and its members.

[48] Jason Taliadoros, ‘Natural Law and Legal Obligation: Pre-Modern Understandings of Ius Naturale’. Paper presented at the 2011 Conference of Australian Society of Legal Philosophy, 29–31 July 2011, Brisbane/Qld, p 4.

[49] Wilfried Hartmann and Kenneth Pennington, The History of Medieval Canon Law in the Classical Period (Washington/DC: Catholic University of America Press, 2008), pp 1140–1234.

[50] Taliadoros, above note 48, p 3.

[51] Ibid, p 64.

[52] Berman, above note 1, p 73.

[53] See Edward S Corwin, The ‘Higher Law’ Background of American Constitutional Law (Ithaca/NY: Cornell University Press, 1955). See also Charles J Antieau, ‘Natural Rights and the Founding Fathers — The Virginians’ (1960) 17 Washington and Lee Law Review 43-79.