QuestionsMust we always obey the law?An unjust law is not truly a law
Must we always obey the law?Conscience first

An unjust law is not truly a law

Lex iniusta non est lex

Thomas Aquinas 1225 · 1274Faith and reason cannot contradict each other.
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What it claims

An unjust law does not bind in conscience (except to avoid scandal or disturbance), and if it orders against divine law, it must in no way be observed.

Why

A law that departs from reason and the common good is more an act of violence than a law: it does not bind the conscience.

The same idea through history
Sophocle441 BCAntigone appeals to unwritten laws of the gods against Creon's edict (ll. 450-455). This is the image Aquinas systematises.
Augustine of Hippo395On Free Choice of the Will I, 5: a law that is not just does not seem to him to be a law. Aquinas cites this sentence to ground his thesis.
Thomas Aquinas1270Summa Theologiae, I-II, q.96, a.4: the version presented here. It presupposes his theory of natural law, presented in another argument.
Martin Luther King Jr.1963Letter from Birmingham Jail: King cites Augustine and Aquinas to say that an unjust law is one not rooted in eternal law and natural law.
In short

"An unjust law is not truly a law" is an argument by Thomas Aquinas (Thomas Aquinas, Summa Theologiae, I-II, q.96, a.4 (c. 1270), 1270) on the question "Must we always obey the law?" (answer: conscience first). Its claim: An unjust law does not bind in conscience (except to avoid scandal or disturbance), and if it orders against divine law, it must in no way be observed. According to AskPhilosophy's logic check (Z3 solver), the conclusion does follow from its 3 premises. AskPhilosophy presents 3 objections, 3 of them with a documented reply, and the original text behind each premise, checked word for word.

Thomas Aquinas, Summa Theologiae, I-II, q.96, a.4 (c. 1270)
Valid

Must we always obey the law? For Thomas Aquinas, it depends on what a law is. A law is not only an order backed by force: it is a rule of reason directed to the common good. When it departs from that (because the ruler thinks of his own profit, exceeds his power or spreads burdens unfairly) it keeps the appearance of law but resembles an act of violence. It then does not bind the conscience, unless disobeying would cause a greater scandal or disturbance. And when it orders what God forbids, it must never be observed. The P1 to P3 → C form is our reconstruction.

P1A law is an ordinance of reason for the common good, made by him who has care of the community, and promulgated. A just human law draws from the eternal law its power to bind the conscience.
P2A law is unjust when it is contrary to the common good, in its end (the ruler seeks his own profit), its author (he exceeds his power) or its form (burdens are spread unequally). Such a law is an act of violence rather than a law, for, as Augustine says, a law that is not just seems to be no law at all.
P3A law that does not truly have the nature of law does not bind in conscience, except perhaps to avoid scandal or disturbance.
Therefore an unjust law does not bind in conscience (except to avoid scandal or disturbance), and if it orders against divine law, it must in no way be observed.

click a premise to reject it

If you accept every premise, logic forces you to accept the conclusion. So the debate is about the premises.
Checked by a logic solver: the premises cannot all be true while the conclusion is false.
The argument is valid. Only a law that truly has the nature of law binds the conscience, apart from scandal (P1 and P3). But an unjust law is an act of violence, not a law (P2). Its strength is to distinguish moral obligation from de facto obedience: one can yield to force without owing it obedience. Its weak point is that someone must decide when a law is 'unjust', and who decides.
Analogy. A 'No entry' sign put up by a thief in front of your own house: it has the shape of a sign but not the nature of a rule. It does not oblige you to stay outside, even if you might prefer to avoid a quarrel.

Objections

3

Each objection is a conversation: click to see who replies and where the debate stands.

Objection anticipated by Aquinas 1270 · attacks premise 1

It would seem that human law does not bind man in conscience: an inferior power has no jurisdiction in a court of higher power. But the power of man, which frames human law, is beneath the Divine power. Therefore human law cannot impose its precept in a Divine court, such as is the court of conscience.

Thomas d'Aquin replies
Thomas Aquinas 1270 · Aquinas's reply in the article
Aquinas replies, with Saint Paul, that all human power is from God: whoever resists authority, in matters within its scope, resists the ordinance of God and becomes guilty in conscience. The objection fails for just laws.
The debate goes on (our summary) This reply maintains the duty to obey just laws. The question remains who judges that a law is unjust. Aquinas does not answer that in this article; he only limits the obligation by the criteria of end, author and form (our summary).
Where the debate stands (our summary)The objection says conscience answers to God alone. Aquinas replies that human authority comes from God, so it binds in conscience when it is just. The basic disagreement remains: who judges that a law has lost that authority?
Thomas Hobbes 1651 · attacks premise 2

Hobbes refuses to speak of an unjust law: by a good law, he says, I mean not a just law, for no law can be unjust. The law is made by the sovereign power, and all that is done by such power is warranted and owned by every one of the people; that which every man will have so, no man can say is unjust. It is with the laws of a commonwealth as with the laws of gaming: whatsoever the gamesters all agree on is injustice to none of them.

a defender replies
see the argument
No reply possible: objection raised in 1651, after the author's death (1274).
G
Gustav Radbruch defender of the argument · 1946
After Nazism, Radbruch argues that positive law keeps priority in principle, even if unjust, for the sake of legal certainty, except when the injustice reaches an intolerable degree: then the statute must yield to justice. When a statute does not even aim at justice and deliberately betrays equality, it is not law: it is 'statutory lawlessness'.
H
H. L. A. Hart 1958
Hart answers that saying of a monstrous rule that it is not law is a poor way to pose the problem. It is better to say: this is law, but too evil to be obeyed. That keeps the idea that the law exists while leaving us to judge, morally, that it can be broken. Merging law and morality would lead to confusion.
See the rest of the debate (1)
L
Lon L. Fuller defender of the argument · 1958
Fuller, in the same issue of the journal, answers Hart that law has an internal morality (rules that are public, general, clear, stable, not retroactive): a system that gravely violates them ceases in a sense to be law. The separation of law and morality is therefore not as clean as Hart says.
The debate goes on (our summary) Hobbes writes before the modern quarrel, and Aquinas before Hobbes: neither answers the other. The contemporary discussion (Radbruch, Hart, Fuller) takes up the question afresh, after the laws of the Third Reich. It does not settle it, and many jurists today hold intermediate positions (our summary).
Where the debate stands (our summary)Hobbes: no law is unjust, because the sovereign speaks for all. Radbruch: an intolerably unjust law is not law. Hart: call it law, but do not obey it. Fuller: a system without internal morality is not fully law. The basic disagreement remains: should 'law' be a descriptive or a moral word?
T
The Laws (Plato's character) 390 BC · attacks the conclusion

An objection drawn from the Crito, written long before Aquinas. According to Plato, the Laws tell Socrates he must either convince the city or do what it orders, including suffering what it orders him to suffer, without giving way. If he flees after even an unjust condemnation, he destroys, as far as he can, the laws and the city.

a defender replies
see the argument
No reply possible: objection raised in -390, after the author's death (1274).
Martin Luther King defender of the argument · 1963
King takes up Augustine and Aquinas to distinguish just and unjust laws, and defends disobeying the latter, but in a particular way: whoever breaks an unjust law must do so openly, lovingly, and with a willingness to accept the penalty. He sees in this attitude the highest respect for law, and names Socrates among those who practised civil disobedience.
The debate goes on (our summary) King takes a middle path: like Socrates he accepts the penalty; like Aquinas he refuses to give the unjust law the force of law. But a difference remains: Socrates lets himself be executed without breaking the law that condemns him, whereas King deliberately breaks a law he judges unjust (our summary).
Where the debate stands (our summary)The Laws of the Crito ask to obey or convince. King and Aquinas distinguish just and unjust laws, and King adds that one must accept the penalty. The basic disagreement remains: does accepting the penalty of an unjust law recognise it or honour it?