The natural law tradition is the oldest continuous tradition in Western legal thought. Its central claim is that law is not merely what the sovereign commands; it is constrained by principles that exist prior to and independent of any human enactment. An unjust law — one that violates these principles — is defective as law, not merely as policy.
The tradition begins with Aristotle’s distinction between natural justice (universal and unchanging) and conventional justice (variable by community). Cicero extended this into a theory of universal law grounded in reason: “True law is right reason in agreement with nature; it is of universal application, unchanging and everlasting.” Aquinas systematized the tradition within Christian theology. He distinguished eternal law (God’s rational ordering of the universe), natural law (the portion of eternal law accessible to human reason), human law (specific rules enacted by human authorities), and divine law (revealed in scripture). Human law, in this framework, must conform to natural law — and when it doesn’t, it loses its binding force.
The early modern period secularized natural law without abandoning it. Grotius argued that natural law would hold “even if we should concede that there is no God” — grounding it in the rational requirements of social life rather than divine will. Locke derived natural rights (life, liberty, property) from a hypothetical state of nature, providing the theoretical foundation for constitutional government and the American and French revolutions. Kant reformulated natural law as the requirements of practical reason: legal order must respect the autonomy and dignity of rational agents.
In the twentieth century, natural law reemerged in response to positivism’s inability to condemn Nazi law on legal grounds. If law is simply what institutions enact, then the Nuremberg Laws were valid law. Lon Fuller’s The Morality of Law (1964) argued that law has an “inner morality” — procedural requirements (generality, publicity, prospectivity, consistency) without which a system of rules doesn’t qualify as law. John Finnis’s Natural Law and Natural Rights (1980) revived Thomistic natural law in analytical philosophical terms, arguing that human rights are grounded in objective goods — life, knowledge, friendship, practical reasonableness — that all societies must respect.
Critiques and limitations
Positivists charge that natural law confuses law with morality. The question “what is the law?” is an empirical question about institutional facts; the question “what should the law be?” is a moral question. Natural law conflates them, and the conflation is dangerous in both directions: it can legitimate unjust positive law by calling it “natural” or delegitimize just positive law by claiming it violates an unverifiable higher standard.
From the left, the concern is that natural law’s appeal to universal principles is politically conservative — it treats existing arrangements as natural rather than historical, and its invocations of “nature” have been used to defend slavery, patriarchy, and the criminalization of homosexuality. The content of “natural law” tends to reflect the commitments of whoever is doing the invoking.
From postcolonial perspectives, natural law’s universalism is suspect. The “universal” principles the tradition identifies are products of specific European intellectual traditions — Greek philosophy, Roman law, Christian theology, Enlightenment rationalism. Their claim to universality is itself a political claim, historically deployed to justify the subordination of non-European legal systems as mere “custom” rather than real law.
Key texts
- Aristotle, Nicomachean Ethics, Book V (c. 350 BCE)
- Thomas Aquinas, Summa Theologica, I-II, Questions 90–97 (c. 1270)
- Hugo Grotius, On the Law of War and Peace (1625)
- John Locke, Second Treatise of Government (1689)
- Lon Fuller, The Morality of Law (1964)
- John Finnis, Natural Law and Natural Rights (1980)
See also
- Legal Positivism — the tradition that defines itself in opposition to natural law
- Natural law — the term entry for the concept
- Rights — the concept natural law theorists claim to ground
- Legitimacy — the quality natural law claims to provide independent of positive enactment
- Customary law — another non-positivist source of legal obligation