the burden of proof when it comes to providing defences is up to the defendant when they make a claim. since that burden of proof exists and assuming a defendant can prove that they didn't know the applicable law , would it be unjust to not allow this defence ? also is it even possible to prove this kind of defence ?

  • 1
    Despite the ignorantia juris non excusat principle, ignorance, when it can be substantiated, is often taken into consideration, especially at sentencing. There are even legal exceptions when pleading ignorance is allowed as a defense, e.g. for new laws (Canadian hunters were acquitted when a law was changed while they were in the wilderness) or for "knowing violations", where "specific intent" is an element of the crime.
    – Conifold
    Commented Mar 30, 2023 at 3:59
  • how does specific intent relate to mistake of law ? Commented Mar 30, 2023 at 9:13
  • Some crimes require scienter, a culpable state of mind, or intent to bring about specific harmful result. Those unaware that their conduct is unlawful may lack such state or intent, see criminal intent.
    – Conifold
    Commented Mar 30, 2023 at 9:58
  • Which legal system? Different systems will have very different answers to this question.
    – Boba Fit
    Commented Mar 30, 2023 at 13:45

2 Answers 2


I'm going to restate the problem here for clarity as I discuss the analysis.

The Case: We have a person, say Adam, who has committed some crime X. When brought to court, Adam says he did not know that X was a crime when he was doing X. This is the Ignorance Defence.

Is The Ignorance Defence legal?

No, generally this falls under a legal principle that states that ignorance of the law is not an excuse for breaking it.

(There are specific cases, however, where this defence would work, where the law states things like "willingly")

Is it unjust to not allow the Ignorance Defence?

There's no clear consensus here. However, there are a number of philosophical arguments why the Ignorance Defence should not be allowed.

The core of your question is probably based around the idea of Epistemic Vice.

Epistemic Vice is the idea that you might be responsible for action X and responsible for not knowing that action X was wrong. The claim is that there are certain facts that agents are obligated to know. (Usually with the caveat that the agent must have had the reasonable capacity to know these facts).

So the argument against the Ignorance Defence would go:

  1. Adam had an obligation to know that X was illegal.
  2. If X is illegal, then X is wrong.
  3. Adam did X.
  4. If Adam had an obligation to know that X was wrong, and Adam did X, then Adam is guilty of doing X.

C: Therefore, Adam is guilty of doing X.

Obviously the two caveats here are Premise 1 and Premise 4.

General justifications for Premise 1 are:

  1. Normative justifications: We live in a society where knowing the laws is a social norm. By extension, knowing the laws is your duty as a citizen governed by these laws and thus you have failed your duty by not knowing them, thus justifying Premise 1.
  2. Presumption: We could also claim that Adam can be presumed to know the law based on his broader knowledge. We assume that Adam knows that he lives in a society with laws. We also assume Adam knows that these laws govern all behaviour, including his own, and that they carry penalties for violating them. Thus, we can reasonably assume that Adam has strong incentives for knowing the law if he wants to avoid the penalties. Thus, we can reasonably assume that Adam knew the law. (If Adam doesn't, then it is probably because he was bad at reasoning, or because he was lazy, etc. In these cases, he has committed an epistemic vice. Either that, or that he did not want to avoid the penalties, in which case I suppose punishing him is justified because he accepts the penalties for his actions willingly.)

(Note: Obviously if Adam does not know that laws exist in the first place, then he probably falls outside these arguments, but that will be beyond the scope of this answer.)

Premise 4 feeds into the broader philosophical debate of whether Epistemic Vice even exists and you can read about Revisionism (that opposes Premise 4) and the responses to it here.

There are also historical arguments, both in the flavour of epistemic vice and outside it. I've summarised some key ones here:

  1. Christian Theistic Argument: Leviticus 5:17 makes it clear that you are guilty of a crime before the Lord even if you did not know it was a crime. (Though it's unclear if this makes the crime less severe). This seems to be an a priori argument, it is this way because the Bible says it is.
  2. Natural Law Argument: Some ancient Greek and Roman thinkers, like Cicero, would probably argue that legal systems are bound to and derived from cultural and natural laws. This leads to a presumption of knowledge, where just by living in society you are inevitably exposed to these laws and ideas.
  3. Duty to Participate: A lot of thinkers, from Socrates, to Social Contract Theorists, are committed to some idea of political obligation that could reasonably answer this by saying that knowledge of the law is part of this obligation. Social Contract Theorists could justify opposition to the Ignorance Defence by saying that the people, as a whole, have consented to the laws and so they are enforceable. (i.e. someone else agreed to the law for you).
  4. Utilitarian Argument: Obviously there's the Utilitarian argument that allowing the Ignorance Defence means allowing a defence that is difficult to prove or refute, and in addition incentivises people to not seek out the legality of their actions.

At its core, the answer I find most compelling is a combination of (1) the intuitive force of Epistemic Vice (if you did something that really hurt someone, it seems reasonable to say that not knowing it would hurt them doesn't absolve you of responsibility), (2) the idea that presumption of legal knowledge seems reasonable, and (3) the argument that one's social contract with the government includes a duty to educate oneself about the legal system at the very least.

For opposing arguments that support the Ignorance Defence, I'd recommend looking at Revisionism (linked above) and maybe checking out Matthews' arguments around the legal status of the ignorance defence.


In principle, it is unjust to demand that average people learn every intricacy of the legal system. Indeed, it is impossible, since no one can predict in what way a judge or jury would rule in a particular case. It is impossible for a person to simply "know every law" and act accordingly.

In practice, laws are not arbitrarily written by aliens from Mars. They often codify things that are already commonly accepted in society. For example, few people would sincerely claim that it's news to them that theft or murder is illegal. You would assume that it would be even without reading a single page of criminal code.

Often, as you get into more commercial areas, unjust laws become more common. For example, hidden fees, counterintuitive "gotchas" about parties surprisingly receiving ownership, and so forth. I would consider these outside the scope of the question's "ethics" tag, because these typically arise from the corrupting influence of moneyed interests on government, or the government itself becoming corrupt in the name of increasing its revenue. However, even so, the authorities enforcing such laws rarely use them as pure "gotchas". They will usually attempt to contact you, and inform you that you have not done something required by some law, and give you opportunities to comply and avoid punishment or receive a smaller one.

The courts are normally a last resort, after you have actively refused multiple opportunities to comply. Even with simple criminal acts, if the police see you trying to engage in illegal activity, they will attempt to discourage you (where practical) rather than watch you do the bigger crime and bring you in. So people ignorant of the law may often be reminded about the law by the authorities, when they are about to break it. So while many people do not know the law, not all of those would remain ignorant by the time they end up in court for breaking a law - they would have been informed at some point. This is obviously not a perfect system, many people end up in court out of the blue due to infractions they knew nothing about, but it's an important caveat.

As for allowing ignorance as a defense, the practical problem is that it's a defense that is almost always applicable. Everyone would plead it. And it's very difficult to prove that someone does know something. If such a defense would allow, it would get even worse, as prospective criminals would actively avoid speaking about laws or appearing to learn about them. It would also encourage honest people to not learn the laws, which is also a negative. Realistically, you would have to periodically require that people sign a statement to the effect of "I know these laws and promise to follow them", which you can then use in court to pre-empt the ignorance defense. Yet this is already the case for many non-obvious laws. For example, when receiving a driver license, you are required to attest that you've learned all the complexities of traffic law - in fact you are tested on it.

Moreover, rather than requiring endless statements, many laws already have a builtin standard of "what a reasonable person would do". In this case ignorance of the law by itself is not meaningful, since it could be claimed that even if ignorant of the law, you could have still acted "reasonably". Of course the definition of what is reasonable is yet another can of worms, but for what it's worth, it's there.

So I think that, while in principle it seems fair to not punish people for breaking rules they didn't know, in practice there are already multiple safeguards against such "unfair" punishments, and moreover there are many practical problems caused by allowing ignorance as a defense. For this reason I think the injustice of not allowing it is actually quite small, compared to how it would seem per se.

If anything, I would say the solution here is that it should be part of the government's mandate to ensure that the public is sufficiently educated in what the law expects of them, through effective channels such as primary school, public broadcast and government-sponsored free legal advice. Meanwhile, the legislature should be discouraged from making laws that are complex and confusing, thereby also reducing how much work the fore-mentioned "education" will be. Unfortunately, this does not appear to be a common trend as of yet.

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