Natural law theory 

Where legal positivism separates law from morality, natural law theory denies that the separation can be complete. This article — the second in the jurisprudence-and-legal-theory module — sets out the natural law tradition as the standard response to positivism taught in first-year jurisprudence, and explains the two broad forms it takes.

The core natural law claim

The central claim of natural law theory is that law is connected to morality in a way that goes beyond frequent overlap: a rule that is sufficiently unjust is, on this view, deficient as law — and on stronger versions of the theory, not really law at all, however properly it was enacted. This is often summarised in the maxim "an unjust law is not a law" (lex injusta non est lex), though natural lawyers differ sharply on how far to push that conclusion. A moderate natural lawyer might say an unjust rule is valid but defective, giving officials and citizens a strong moral reason to change or resist it; a stronger version denies the unjust rule the status of law altogether, so that officials applying it are not really applying "law" in the fullest sense.

Natural law theory is not the claim that judges should decide cases by their own sense of morality instead of by legal rules. It is a claim about what law is — that legal validity cannot be assessed by source alone, because a legal system's rules answer, at least in part, to a moral order that exists independently of human enactment.

Classical natural law: reasoning from a moral order

The classical tradition, associated with reasoning in the style of Aquinas, treats law as ultimately derived from, and answerable to, a rational moral order — sometimes framed in religious terms, sometimes in terms of practical reason directed at genuine human goods. On this account, human-made law (positive law) is a specification of more general moral principles for a particular community, and a purported law that flatly contradicts those principles is a corruption of law rather than a valid instance of it. Classical natural law is less concerned with the mechanics of legal systems than with grounding law's authority in something beyond the fact of enactment.

Modern natural law: Fuller's inner morality of law

Twentieth-century natural lawyers largely moved away from grounding law in a comprehensive moral or religious order, toward more modest claims about what law necessarily requires to function as law at all. The best-known modern version, associated with Lon Fuller, argues that law has an inner morality — a set of procedural requirements a system of rules must substantially satisfy to count as a legal system at all: rules must be reasonably general, publicly promulgated, prospective rather than retroactive, reasonably clear, non-contradictory, possible to comply with, relatively stable over time, and actually administered as announced. A regime that consistently fails these requirements — secret rules, rules that change after the fact, rules no one could obey — is not simply making bad law; on this account it risks failing to make law at all, whatever it enacts.

This procedural version of natural law is attractive to students because it does not depend on contested claims about a substantive moral order. It offers a test that looks almost administrative, yet it still insists — against positivism — that some moral or evaluative content is built into the very idea of law.

Why this is the standard foil to positivism

Natural law theory is taught alongside positivism because the two give directly opposed answers to the same question raised in the positivism article: does an unjust or defective enactment count as law? Positivism says a valid rule is law regardless of its moral content; natural law, in one version or another, denies that validity and moral merit can be so cleanly separated. Neither theory is simply "right" — the debate exists precisely because both capture something real about how lawyers actually talk: we say "that's the law, however unjust" (a positivist sentence) and we also say "that's not really law, that's just tyranny dressed up as law" (a natural law sentence). Recognising which sentence an argument is making is the point of studying both.

Applying this in a problem question

1. When a problem invites you to criticise a rule as unjust, decide whether you are making a natural law argument (the rule is deficient as law) or a policy argument (the rule is bad and should be reformed) — they support different conclusions.
2. If invoking Fuller's inner morality of law, test the rule against specific procedural failures — secrecy, retroactivity, unintelligibility, unadministered promises — rather than a general sense of unfairness.
3. Use natural law reasoning to strengthen an argument that a purported exercise of legal power should not be recognised, where the defect goes to basic legality rather than to policy preference.
4. Always identify which version of natural law you are relying on — moderate (valid but defective) or strong (not law at all) — since the strength of the conclusion depends on it.