Introduction
The relationship between law and morality is one of the most enduring questions in legal philosophy. A central debate revolves around whether law must have a moral component to be considered valid. Legal positivists, for instance, argue for the separation of law and morality, suggesting that a law's validity depends on its source and how it was made, not its moral content. By contrast, the Natural Law school of thought proposes that there is an essential, unbreakable link between law and morality. This essay will explain the statement that Natural Law provides the foundation for understanding this relationship. It will argue that the core principle of Natural Law theory is that human-made law is subordinate to a higher moral order, and that an ‘unjust law is no law at all’. This position will be explored through the works of key thinkers such as St. Thomas Aquinas and Lon Fuller, before illustrating its continued relevance through contemporary examples like the development of international human rights and debates surrounding euthanasia.
The Classical Foundation: St Thomas Aquinas
The Natural Law tradition has a long history, with roots in ancient Greek philosophy, but its most influential classical formulation for Western legal thought comes from the theologian St. Thomas Aquinas. Aquinas developed a comprehensive structure of law which placed morality at its heart. In his Summa Theologica, he outlined a hierarchy of four types of law: Eternal Law, Natural Law, Divine Law, and Human Law (Aquinas, 1947). For the purposes of legal theory, the crucial relationship is between Natural Law and Human Law.
Eternal Law represents God's perfect plan for the universe, which is unknowable in its entirety to humans. Divine Law is the part of Eternal Law revealed through scripture. Natural Law is the part of Eternal Law that humans can understand through the application of reason. Aquinas argued that humans have a natural inclination to do good and avoid evil, to preserve life, and to live in society. By using their reason, people can discern these fundamental principles of morality which form the Natural Law.
This leads to the most important part of Aquinas's theory for understanding the law-morality connection: the status of Human Law. Human Law is the positive law of the state—the statutes, rules, and regulations that govern society. According to Aquinas, for Human Law to be valid, it must be derived from and consistent with Natural Law. He famously declared that "an unjust law is not a law" (lex injusta non est lex). This does not necessarily mean that one should disobey any law that seems unfair. However, it does mean that a law that fundamentally violates the principles of natural justice—for instance, one that commands murder or violates human dignity—lacks the moral authority that true law should possess. Therefore, in Aquinas’s view, law is not merely an instrument of power; it is an ordinance of reason for the common good, and its legitimacy is founded upon its moral correctness.
A Modern Procedural Approach: Lon Fuller
While Aquinas provides a classical, substantive account of Natural Law based on religious and moral ends, modern thinkers have re-imagined the theory. Lon Fuller, a 20th-century legal philosopher, offered a secular, procedural version of Natural Law. He was less concerned with the substantive moral content of laws and more interested in the process by which laws are made and administered. In his book, The Morality of Law, Fuller argued that for a system of rules to be properly called a 'legal system', it must adhere to what he called an "inner morality of law" (Fuller, 1969).
Fuller proposed eight principles that constitute this inner morality. These are that laws must be: (1) general, (2) publicly promulgated, (3) prospective (not retroactive), (4) clear and understandable, (5) not contradictory, (6) possible to obey, (7) reasonably stable over time, and (8) applied and administered as they are written. Fuller famously used the hypothetical example of a monarch named Rex who failed to abide by these principles. By creating secret laws, retroactive laws, and contradictory rules, King Rex failed not just to make good law, but to make law at all.
Fuller’s theory provides a different but equally strong foundation for the law-morality link. For him, a legal system that completely disregards these principles of procedural fairness is not just a bad legal system; it is not a legal system at all. This is because such a system would be an affront to the dignity of the individuals it governs, treating them as objects to be managed rather than as responsible agents capable of following rules. Therefore, morality is connected to law not just through its ultimate goals (as in Aquinas), but through the very procedures and forms that make law possible.
Natural Law in Contemporary Society
The ideas developed by Natural Law thinkers are not merely historical or theoretical curiosities; they continue to influence and provide a framework for understanding contemporary legal issues. The most prominent example is the modern human rights movement. The Universal Declaration of Human Rights (UDHR) and subsequent treaties like the European Convention on Human Rights (ECHR), which is incorporated into UK law by the Human Rights Act 1998, are built on a fundamentally natural-law premise. These instruments assert that all individuals possess certain "inalienable" rights simply by virtue of being human. This idea directly reflects the Natural Law concept that there are moral principles that are universal and pre-exist any positive state law. When domestic laws in a country permit torture or deny a fair trial, for example, they can be challenged as violating a higher moral and legal standard embodied in human rights law. The Nuremberg trials after World War II also famously relied on natural law reasoning, prosecuting Nazi officials for "crimes against humanity" even though their actions may have been legal under the positive laws of the Third Reich. The argument was that these state laws were so morally abhorrent that they could not be considered valid law.
Another contemporary example arises in debates concerning the beginning and end of life. Issues such as abortion and euthanasia involve deep moral disagreements. In the UK, the law on assisted dying remains restrictive, as confirmed in cases like R (Nicklinson) v Ministry of Justice [2014] UKSC 38. Arguments against legalisation are often based on the principle of the 'sanctity of life', which is a powerful moral claim that many believe should be reflected in the law. This reflects a natural law perspective: that positive law should uphold fundamental moral values, such as the inherent value of human life. While Parliament could change the law, the debate itself is framed around whether the law should align with a particular moral viewpoint, demonstrating how the Natural Law perspective provides the language and foundation for such arguments.
Conclusion
In conclusion, the statement that the Natural Law school provides the foundation for understanding the relationship between law and morality is fundamentally correct. From the classical theories of Aquinas, which insist that human law must conform to a higher moral order to be valid, to the modern proceduralism of Fuller, who argued that a legal system must possess an 'inner morality' to even exist, Natural Law theory has consistently placed the link between law and morality at the centre of its analysis. It provides a powerful framework for critiquing positive law, asserting that law is not simply whatever a sovereign commands. As seen in the development of international human rights and in ongoing socio-legal debates, the idea that there are moral standards against which law can and should be judged remains a vital and influential perspective in jurisprudence and in society. While legal positivism offers an alternative account, it is the Natural Law tradition that insists on asking not just "what is the law?", but also "what should the law be?".
References
Aquinas, T. (1947) Summa Theologica. (Fathers of the English Dominican Province, Trans.). Benziger Bros. (Original work published 13th Century).
Fuller, L. (1969) The Morality of Law. Revised edition. Yale University Press.
R (Nicklinson) v Ministry of Justice [2014] UKSC 38.
Wacks, R. (2020) Understanding Jurisprudence: An Introduction to Legal Theory. 6th edition. Oxford University Press.
