Natural law from the Greeks to Kant

1. What “natural law” claims

Every natural law theory makes two claims. The first is about morality: there are principles of right conduct that are not made by any legislator and can be known by reason. The second is about law: positive law draws its authority, at least in part, from agreement with those principles. Theories differ on the source of the higher law, on who can know it, and on the consequence of conflict. The strong version says an unjust law is not law at all; the weaker version says it is law but lacks the moral claim to obedience. This distinction is the key to the twentieth-century debate between Hart and Fuller (see the note on Fuller and the revival).

Friedmann’s observation, a useful opening line, is that natural law has served opposite purposes at different times: to defend the existing order (medieval Church, absolute monarchy) and to attack it (the American and French revolutions, the Nuremberg trials). Its content has been variable; its function is constant: to judge positive law by a standard outside it.

2. The Greeks

• The pre-Socratics and Sophists distinguished physis (nature) from nomos (convention). Some Sophists used the distinction to say that law is only the will of the stronger, which forced later thinkers to defend justice by nature.
• Sophocles’ Antigone is the literary starting point: Antigone buries her brother against Creon’s edict, appealing to unwritten and unchanging laws of the gods.
• Socrates taught that virtue is knowledge and that there is a justice discoverable by reason, yet he accepted death under Athenian law rather than escape. The episode is cited both for natural law and for the duty to obey.
• Plato (Republic, Laws) placed justice in a harmony of the soul and the city, ruled ideally by philosopher-kings; in the later Laws he accepted the rule of law as second best.
• Aristotle (Nicomachean Ethics, Politics, Rhetoric) separated natural justice, which has the same force everywhere, from conventional justice, which depends on enactment. Man is a political animal whose nature is fulfilled in the polis. He also gave the theory of equity as a correction of law where it fails by reason of its generality.

3. The Stoics and Rome

The Stoics (Zeno, later Seneca, Epictetus, Marcus Aurelius) located natural law in universal reason (logos) shared by all men. Because reason is common, all men are equal citizens of one world-city. This cosmopolitan idea had a direct influence on Roman law, which is why the 2014 paper asks you to comment on the Stoic influence on the Roman legal system.

• Cicero (De Re Publica, De Legibus) gave the classic statement: true law is right reason in agreement with nature, universal, unchanging and everlasting; it cannot be altered by senate or people, and a law contrary to it is not truly law.
• Roman jurists worked with three categories: jus civile (law for citizens), jus gentium (law common to all peoples, developed by the praetor peregrinus for disputes involving foreigners), and jus naturale. Gaius treated the jus gentium as natural reason’s law among all men; Ulpian defined jus naturale as what nature has taught all animals.
• The practical effect. Stoic ideas softened the formalism of the jus civile: good faith (bona fides) in contracts, informal consensual contracts, the protection of slaves against extreme cruelty, and the jurists’ statement that by natural law all men are born free. The jus gentium became the vehicle by which reason entered the positive law.

4. The Jewish tradition

The Hebrew conception is of law as divine command and covenant. The Torah, including the Decalogue, is given by God to a people bound to him by covenant; the king is under the law, not above it, and the prophets judge rulers by it. Law is revealed rather than discovered by reason, but it shares with natural law the idea of a standard binding every human authority. Through the Old Testament it passed into Christian thought and into the medieval conviction that the ruler is subject to a higher law.

5. The Middle Ages

The Church Fathers, chiefly St Augustine (City of God), set the earthly city against the city of God and taught that a state without justice is a band of robbers. Law became an instrument of divine order.

St Thomas Aquinas (Summa Theologiae) produced the synthesis of Aristotle and Christian theology that still underlies Catholic natural law. He classified law into four kinds:

1. Eternal law: God’s reason governing the whole universe.
2. Natural law: the participation of the rational creature in eternal law, known through reason. Its first precept is that good is to be done and evil avoided; from it follow inclinations to preserve life, to reproduce and educate offspring, to know the truth and live in society.
3. Divine law: revealed in scripture, to guide man to his supernatural end.
4. Human law: the particular applications of natural law made by those in charge of the community, for the common good, and promulgated.

Human law that departs from natural law is, for Aquinas, a perversion of law rather than law, but he did not license disobedience freely: an unjust law may still be obeyed to avoid scandal or disorder, unless it commands what divine law forbids. The view that an unjust law is no law at all is usually traced to Augustine and repeated by Aquinas; Finnis later argued that it was never meant as a statement that such a rule has no legal existence.

6. Renaissance and Reformation

The Reformation broke the Church’s monopoly on interpreting natural law, and the rise of nation-states and of science pushed thinkers to ground natural law in reason and human nature rather than theology. The 2014 paper asks for the growth of natural law in this period; the sequence is:

• Hugo Grotius (De Jure Belli ac Pacis, 1625) secularised natural law. He argued that it would hold even if, per impossibile, God did not exist, because it springs from man’s social nature (appetitus societatis). Its core rules (keep promises, restore what belongs to another, make good damage done by fault) became the basis of international law; Grotius is called its father.
• Thomas Hobbes (Leviathan, 1651). In the state of nature life is “solitary, poor, nasty, brutish, and short.” Laws of nature are rational rules of self-preservation, and they lead men to surrender their rights to an absolute sovereign. Natural law here supports absolutism and points towards positivism.
• John Locke (Two Treatises of Government, 1689). The state of nature is governed by a law of reason that protects life, liberty and estate. Government is formed by consent to protect these natural rights; if it violates them the people may resist. Locke is the source of the American Declaration of Independence (1776) and of the labour theory of property (see Legal Concepts, property).
• Jean-Jacques Rousseau (The Social Contract, 1762). Sovereignty lies in the general will of the people, which is always directed to the common good. The idea fed the French Declaration of the Rights of Man and of the Citizen (1789).
• Pufendorf, Vattel and Montesquieu carried the rationalist method into public and international law; Montesquieu also opened the sociological approach.

7. Kant

Immanuel Kant (Groundwork of the Metaphysics of Morals, 1785; The Metaphysics of Morals, 1797) moved the foundation from human nature to the autonomous rational will. The moral law is the categorical imperative: act only on a maxim that you can at the same time will to become a universal law; and treat humanity, in yourself and in others, always as an end and never merely as a means.

Kant separated morality (inner motive, duty for duty’s sake) from law (external conduct, which may be coerced). His universal principle of right holds that an action is right if it can coexist with the freedom of everyone under a universal law. Coercion is justified only as a hindrance to a hindrance of freedom. Law therefore exists to secure equal external freedom, and the just state is a republic based on an idea of original contract. For a short note on “Kant’s theory of law” (2024), state these three points, then add his influence: human dignity in the German Basic Law (Article 1) and in Indian dignity jurisprudence under Article 21, and Rawls’s Kantian interpretation of justice as fairness.

8. Decline in the nineteenth century

Natural law lost ground for four reasons: Hume’s argument that an “ought” cannot be derived from an “is”; Bentham’s attack on natural rights as “nonsense upon stilts”; the historical school’s insistence that law grows from a people’s particular life rather than universal reason; and the success of codification and analytical positivism, which treated law as the command of the sovereign. Its revival after 1900 and especially after 1945 is the subject of the next note.

How this is examined

As an essay: “Define natural law and its applications by different philosophers at different times” (internal, January 2024), “The theory expounded by the Stoics had a great influence on the Roman legal system” and “Discuss the growth of natural law during the Renaissance” (June/July 2014). Arrange the answer by period (Greek, Stoic and Roman, Jewish and medieval, Renaissance, Kant) and end with the decline and revival. As a short note: Kant’s theory of law (March/April 2024), or Aquinas’s four kinds of law.