Lex iniusta non est lex — an unjust law is no law at all. Aquinas strongly supports this principle, drawing on the example of Augustine before him. But is he correct? Is it, in fact, the case that one has no obligation to obey a law that actually is unjust? And, moreover, is there also no obligation to obey a law merely on the basis that one believes it to be unjust? The two are separate and important questions, and I believe that ultimately the answer to them is that for moral reasons there is no obligation to obey a law that is in fact unjust, but that it is difficult to be certain whether a law truly is unjust based on appearances alone. For these reasons, there is a general duty to obey the law irrespective of how it appears to you as an individual.It should be obvious from an Augustinian perspective and a Thomistic perspective that there exists a generalized duty to obey laws that are, in fact, moral. In other words, there is a moral duty to obey laws that cohere with the natural law: a just law is a genuine law. But from this, it does not necessarily follow that an unjust law is not a genuine law. It could be that it is the case that a man, by disobeying the law of his society, is in fact committing a moral wrong in addition to a legal one. It does not seem, though, that there is a compelling reason to think that this is the case. Why should there be anything special about laws instituted by groups of men that affords them legitimately distinct moral status from other imperatives? That is to say, why is there something legitimately morally wrong about my violating law L, independent of the content of L, if L is a governmental imperative rather than a personally-enforced imperative? It does not seem that there should be a reason to draw such a distinction; therefore, it seems that there is no general moral imperative to obey an unjust law.
An objection that might be raised to this position is that men are prone to differ in their moral judgments, even when they attempt to make those judgments in an honest and unbiased way. They are especially prone to bias when one conclusion would be particularly favorable to them and another would be particularly unfavorable. Consider, for instance, drug laws in America: it is very common to hear recreational drug users quote some formulation of “an unjust law is no law” — is it not possible that they are strongly biased? Similarly, consider a sociopath who honestly believes that there is nothing morally wrong with murder, or even consider a moral nihilist — should they be exempt from all laws because they believe that no restrictions are legitimate?
This objection can be answered, though, by drawing a distinction between what can be permitted as a matter of morality and what can be permitted as a matter of practicality. As was mentioned above, there appears to be no reason that unjust laws would be binding. However, man’s moral perception is fallible. It is for this reason that one of the roles that the law serves is that of a moral guideline, demarcating what behavior is or is not acceptable. Because of this, man ought not to assume he “knows better” than those who crafted the law.
So we see that although there may not be a moral duty to obey a law just for the sake of obeying the law, it is generally unjustified to break the law. A single person’s moral intuitions are more likely to be flawed than the law itself is. With that said, if we can point to instances in the past where the law was morally wrong, we can conclude it may have been justified in those cases to break it.