Introduction
The theory of natural law proposed by the 13th-century philosopher and theologian St Thomas Aquinas remains one of the most significant and enduring contributions to legal and political philosophy. Rooted in Aristotelian and Christian thought, Aquinas’s jurisprudence, primarily articulated in his Summa Theologica, presents a comprehensive system where law is fundamentally connected to morality and reason. This essay will explain the core components of Aquinas's natural law theory. It will outline his hierarchy of law, discuss the role of reason in discerning the principles of natural law, and examine the relationship he establishes between morality and man-made (human) law.
The Hierarchy of Law
Aquinas conceived of law not as a single entity but as a four-tiered hierarchy, with each level deriving its authority from the one above it. This structure provides the foundation for his entire legal philosophy (Aquinas, 1947, ST I-II, Q.91).
First, at the highest level, is the Eternal Law (Lex Aeterna). This is God’s rational plan for the universe, the divine reason by which all creation is governed. It is perfect and unchanging, but it is known in its entirety only to God.
Second is the Divine Law (Lex Divina). This is the part of the Eternal Law that has been revealed to humanity through scripture, such as the Ten Commandments. Its purpose is to guide individuals towards their ultimate supernatural end, which is eternal happiness with God.
Third is the Natural Law (Lex Naturalis). This is the part of the Eternal Law that human beings can understand and participate in through the use of their natural reason, without the need for divine revelation. Aquinas argues that because humans are rational creatures, they have a natural capacity to discern the fundamental principles of good and evil that are embedded in the nature of reality.
Finally, at the base of the hierarchy, is Human Law (Lex Humana). These are the specific, positive laws enacted by governments to regulate society. For Aquinas, these laws are only valid and legitimate insofar as they are derived from, and do not conflict with, the principles of natural law.
Reason and the First Precept of Natural Law
The core of Aquinas’s theory is the human ability to use reason to discover the Natural Law. He argued that reason is a divine gift that allows humanity to participate in God's Eternal Law. This process begins with a foundational, self-evident principle that he called the first precept of natural law: "good is to be done and pursued, and evil is to be avoided" (Aquinas, 1947, ST I-II, Q.94, a.2).
From this first precept, other primary precepts can be rationally derived. These correspond to the fundamental goods that humans are naturally inclined to pursue, such as self-preservation, procreation and the education of offspring, living in a community, and seeking knowledge of God. These primary precepts are universal and unchangeable. More specific rules, known as secondary precepts (e.g., 'do not steal' or 'do not murder'), are then derived from these primary principles to address the particular circumstances of human life (Freeman, 2014).
The Connection Between Law and Morality
A central feature of Aquinas’s theory is its rejection of a separation between law and morality. He famously endorsed the principle, often associated with St Augustine, that lex iniusta non est lex—an unjust law is not a law at all. For a human law to be valid, it must conform to the principles of natural law. A law could be unjust, and therefore invalid, in several ways: if it is contrary to the human good, if it is enacted for private gain rather than the common good, or if it exceeds the authority of the law-maker (Wacks, 2020).
This does not necessarily mean that citizens have a right to disobey any law they disagree with. Aquinas suggested that for the sake of public order, one might still have to obey a technically unjust law unless it directly commanded an action contrary to Divine Law. However, such a law would lack true moral authority and would not bind a person’s conscience. Therefore, in Aquinas's view, the moral content of a law is essential to its legal character.
Conclusion
In summary, Thomas Aquinas's natural law theory presents a deeply moralised vision of law. It is structured around a hierarchy of laws originating from God's eternal plan, with human law at the bottom, deriving its legitimacy from its alignment with the higher principles of natural law. These principles are discoverable by all through reason, which directs humanity towards pursuing good and avoiding evil. The famous maxim that an unjust law is no law at all encapsulates his core belief that positive law cannot be divorced from objective morality. While its theological foundations present challenges for application in modern secular states, Aquinas’s theory remains a powerful and influential account of the moral foundations of law.
References
- Aquinas, T. (1947) Summa Theologica. (Trans. Fathers of the English Dominican Province). Benziger Bros.
- Freeman, M.D.A. (2014) Lloyd’s Introduction to Jurisprudence. 9th edn. Sweet & Maxwell.
- Wacks, R. (2020) Understanding Jurisprudence: An Introduction to Legal Theory. 6th edn. Oxford University Press.
