In a previous question's answer, @motosubatsu quoted a passage from James Bigg, Esq. stating this as a "legal axiom."

Where does this perception arise from?

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    In other words, it seems only to be presumed in the same sense as a legal fiction, not presumed to be a genuinely/factually true proposition. Feb 18, 2023 at 14:24
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    you don't have to know all the laws. but you can't claim ignorance if you break it, so it's up to you to make sure you know if you are breaking the law or not
    – njzk2
    Feb 18, 2023 at 14:29

6 Answers 6


James Bigg, Esq. published his Criminal Law Consolidation (from which the “legal axiom” of the question is quoted) in 1865. At that time, the general view of England’s common law system was that it was grounded in the customs of society as a whole. We might therefore turn Bigg’s axiom on its head: It’s not that every Englishman is presumed to know the law; it’s that the law is presumed to be what every Englishman knows.

On this theory, the ordinary Englishman may not in fact know (or even know of) every precedent and statute that exists, but he will know the customs of his country, and know right from wrong. The root of the law is to behave in a way that any decent, right-thinking Englishman would think fair and proper.

Sir William Blackstone’s Commentaries on the Laws of England exemplifies this view. In section III, “Of the Laws of England”, he describes the common law thus (emphasis mine):

I therefore stile these parts of our law leges non scriptae [unwritten laws], because their original institution and authority are not set down in writing, as acts of parliament are, but they receive their binding power, and the force of laws by long and immemorial usage, and by their universal reception throughout the kingdom.

The English were quite proud of their common law, and resisted attempts to codify it. In his “Proposition Touching Amendment of Laws” (1657), Francis Bacon wrote:

Obj[ection] IV. Labour were better bestowed in bringing the common laws of England to a text law, as the statutes are, and setting both of them down in method and by titles.

Resp[onse] …there are more doubts that rise upon our statutes, which are a text law, than upon the common law, which is no text law. But howsoever that question be determined, I dare not advise to cast the law into a new mould. The work which I propound tendeth to proyning and grafting the law, and not to ploughing up and planting it again; for such a remove I should hold indeed for a perilous innovation.

The thinking appears to be that, unlike under the civil law of continental Europe, an English judge was free to innovate if doing so delivered a ruling that reflected a “common” notion of fairness and justice better than the code of laws could. Of course Blackstone did not consider judges as making law; later on in Section III, on the subject of precedent, he says that a judge is:

…sworn to determine, not according to his own private judgment, but according to the known laws and customs of the land; not delegated to pronounce a new law, but to maintain and expound the old one. Yet this rule admits of exception, where the former determination is most evidently contrary to reason; much more if it be contrary to the divine law. But even in such cases the subsequent judges do not pretend to make a new law, but to vindicate the old one from misrepresentation. For if it be found that the former decision is manifestly absurd or unjust, it is declared, not that such a sentence was bad law, but that it was not law; that is, that it is not the established custom of the realm, as has been erroneously determined.

(Amusingly, there is an uncited claim on Wikipedia that Blackstone’s Commentaries was criticised as a covert attempt to codify the common law. I would love to see evidence for this, but a brief search has not turned up any.)

This thinking persisted into the 20th century. Consider Donoghue v Stevenson, where Lord Atkin used the Bible as his basis for saying that manufacturers owe a duty of care to consumers. In this there is an echo of Blackstone’s declaration of the primacy of divine law, but it was also feasible simply because the UK remained, by and large, a Christian country: the precepts of the Bible were the “established custom of the realm”.

This answer is unlikely to be wholly satisfactory, as I don’t have explicit evidence linking the historic view of the common law to the presumption that everyone knows the law. The evidence I have laid out is also focused on common law, but this presumption extends to statute law as well.

Moreover, in the course of writing this answer, I read that this view is no longer popular among legal scholars or practitioners: neither as an account of how the law developed, nor as a rubric to how it should be applied today.

But it was certainly a mainstay of English legal theory at the time when Bigg wrote of this “legal axiom”, and I therefore offer it as an answer to the question. It complements, but does not replace, the pragmatic argument others have put forth, that it is untenable to let ignorance be a ready defence.

  • Is common law in fact to be distinguished from criminal law? I had thought that the former encompasses the latter? Feb 19, 2023 at 8:26
  • I suppose this also goes to explaining the OPCA crowd’s preoccupation with “common law” as it is meant to be one with and a reflection of popular senses of fairness and decency. Often there are proposed to be held “common law courts” to try the traitorous ruling class politicians whose legislations have strayed too far afield and out of touch with this popular common sense of fairness and justice. So the fascination with common law, if viewed in this light, is somewhat more understandable, and even perhaps ina certain sense somewhat founded. Feb 19, 2023 at 8:34
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    @Seekinganswers The distinction is between common law and statute law, with criminal and civil law being a separate dichotomy. I put the wrong word in this answer, but have now corrected it. Feb 19, 2023 at 13:10

I think it's ultimately a natural extension of the basic concept from Roman Law: ignorantia legis non excusat, i.e "ignorance of the law excuses not" and that in order for that to be workable laws can only truly obtain proper binding force when they are promulgated. In simple terms someone can't claim innocence through ignorance because the knowledge of what the law is has been made easily available to them.

Thus the law can operate on the presumption that the people it applies to are aware of them. Indeed there have been occasional instances in history where someone could not have been reasonably expected to know that what they did was against the law and this has been taken into account.

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    And yet, Attorneys spend an enormous amount of time learning what the laws are, and also how to interpret those complex laws. Then a lay-person is held accountable for knowing every law that might apply to them. It seems absolutely impractical for individuals to be knowledgeable of every law that applies to the complexities of daily life. Ignorance may not be an excuse, but it is inevitable when the body of laws is so immense.
    – abelenky
    Feb 17, 2023 at 19:01
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    @abelenky Don't forget that the law then has to be interpreted by a judge for particular cases. For instance, what one jurisdiction calls a deadly weapon another may not, despite both of their statutory laws using the same term "deadly weapon," entirely based on case precedent. Feb 17, 2023 at 19:19
  • @abelenky That's the idea. You're never safe. Feb 17, 2023 at 23:15
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    @abelenky: "It seems absolutely impractical for individuals to be knowledgeable of every law that applies to the complexities of daily life." True, but allowing ignorance to be an excuse quickly becomes even more impractical. You just have to be careful and double-check before doing things that might be unlawful. That's not too hard, since most laws in a civilized society are more or less reasonable.
    – Heinzi
    Feb 18, 2023 at 13:26
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    @abelenky, people in legal professions dedicate most of their time to understanding either the letter of the law or the application of the law. In this context we're only really talking about being aware of the Spirit of the law. Think: "Don't steal, don't start a fight, don't trespass", rather than understanding the intricacies of these law and when they apply. Feb 18, 2023 at 15:52

Because it's a necessity for the system to function

Let's assume for one moment that no person is assumed to know what the law is, because that is the inverse of the premise. This also means that the old brocard of Ignoratia legis non excusat & ignorantia legis neminem excusat (ignorance of the law isn't an excuse/is an excuse for nobody) is to be inverted: Ignorance of the law becomes an excuse for having broken the law.

If ignorance of the law is an excuse to have broken it, every person gets a "free shot" at whatever they want to do, because you can only presume someone knows it because they are put under verifiable notice of the law. Under this setup, there is no way to have an effective legal system.

Only if perfect knowledge of the law is presumed on everybody can the legal system be both effective and fair: no matter if you actually do know or not know, the same verdict for the same act will be reached, if mens rea is not a factor.

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    Re: a "free shot," see also the controversy surrounding qualified immunity for police in the United States. In these cases, US courts assume that the officer doesn't know the law unless there has been a previous case clearly establishing illegality. This a) creates a chicken-and-egg problem because all such cases are dismissed and therefore don't establish illegality for future cases and b) makes the bizarre assumption that while police officers can't reasonably be expected to know the law, they can be expected to be familiar with the entire body of judicial precedent in their jurisdiction.
    – A. R.
    Feb 17, 2023 at 18:52
  • @AndrewRay interesting. so what's the origin of this assumption in the US? Feb 17, 2023 at 18:58
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    @Seekinganswers That alas, is tied up in decades of the aforementioned judicial precedent. There are probably thousands of articles in favor of and opposition to this practice. The origin seems to be based on the assertion that the nature of police work requires officers to routinely act in a legal gray area and that allowing officers to be punished for these actions would lead to a police shortage. As I say, it is a very controversial topic, and I would encourage you to do your own research.
    – A. R.
    Feb 17, 2023 at 19:06
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    Suppose you deny the principle "ignorance of the law does not excuse". This means, rephrased, that you assert that "ignorance of the law is a valid excuse". But that means that everyone would strive to be as ignorant of the law as possible – so much that any knowledge of law would be considered a dangerous thing!
    – printf
    Feb 18, 2023 at 4:27
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    Qualified Immunity in the USA is an entirely judge-made concept that has been used to stop civil action for stealing a drug dealer’s cash (as opposed to turning it in as evidence or for forfeiture) or even sexual assault. It's mind-boggling to see it used for crimes that are malum in se. Why? I might suggest judges wanted to be popular with police or some other political as opposed to legal reason. Feb 18, 2023 at 19:45

First, the laws are generally modeled on a core sense of "right and wrong", and it is expected that every parent, school and foster system will make great effor to teach this to every child. Separately to this, is it modeled on "Do unto others as you would wish them do unto you". So if you reasonably wouldn't want people stealing your car, don't steal cars. If you don't want to see streets covered in litter, don't litter. See also the "reasonable man" metric.

This allows most people to generally navigate life without inadvertently committing criminal offense. It seems to work pretty well.

Of course, life branches into 10,000 different specialties. Say you buy a narrowboat. The onus is on you to learn the laws of operating inside your specialty. If your specialty is commonly accessed by consumers, like owning a narrowboat, there is plenty of stuff to help you, from government education and certifications to private lessons and of course, the Internet. Enforcement is also wise to the earnest but oblivious novice, and will be non-cruel and give you advice. You better listen.

Of course once you start operating at a commercial specialty, like operating an oil tanker or cruise ship, or building a windmill farm or job-matching website, you are expected to retain the counsel of experts - as in hire them, starting with lawyers. Your experts' professional job is to understand the craft-specific laws which no layman would have any reason to know. No excuse is accepted. That gets small business owners in trouble when their business grows dramatically and gains scope their diligence wasn't prepared for - when Port Wynn "match-a-ride" becomes Cornwall Match-a-Ride becomes UK Match-a-Ride and you're still running it with the same in-town accountant and lawyer you started with.

  • Is the match a rude anecdote a true one? Feb 19, 2023 at 8:40
  • err, match a ride* Feb 19, 2023 at 17:05
  • @Seekinganswers No, Port Wynn is the fictitious village from the Doc Martin TV series. Just a hypothetical of how a web business could quickly take off and outgrow their legal due diligence. Feb 19, 2023 at 22:00

It is not presumed that everyone knows the law.

In Kiriri Cotton Co. v. Dewani, [1959] UKPC 27, p. 3-4 Lord Denning said:

It is not correct to say that everyone is presumed to know the law. The true proposition is that no man can excuse himself from doing his duty by saying that he did not know the law on the matter. Ignorantia juris neminem excusat.

It is simply the case that application of the law to you does not require your prior awareness of it.

In a jury trial, it is critical that the judge explain the applicable law correctly to the jury. Failure to instruct the jury on a critical point of law can leave a conviction open to appeal—jurors, like everyone, are not presumed to know the law.


I just spent a year in jail because I didn't know it was a serious crime to change the tires on your car on a Wednesday evening between 8pm and 9pm. Ignorance of the law does not excuse, right? Well, that doesn't happen.

Laws make it illegal for you to do illegal things to others, but it also makes it illegal for others to do illegal things to you. If there is an action that you wouldn't want to suffer from others, and that you think they shouldn't be allowed to do, then you shouldn't yourself do this to others. Even if you don't know for sure that a specific law exists, that cannot be an excuse if it is something that you think should be illegal for others to do. And that's from a normal healthy person's point of view, a sociopath might think otherwise.

Now, ignorance of facts will often be an excuse. Not knowing that it is illegal to exceed the speed limit is not an excuse. Not knowing the speed limit at some bit of public road can be an excuse, depending on the situation. You are supposed to watch out for speed limit signs, so not knowing the speed limit is often not an excuse. But if the sign was invisible because it was behind a tree, that is likely an excuse.

In the end, you don't have to know the laws. You just need to not violate them. If you do out of ignorance, that's just tough for you.

  • I cann not surpress my curiosity regarding the tire changing crime... Feb 19, 2023 at 2:11
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    My children picked up a bird feather in our front yard. We went online to try and identify it and in doing so found out it was a federal crime to possess any wild bird feathers even if they fell off naturally and you just found it on the ground. I don't think any normal person would have guessed that one. fws.gov/lab/featheratlas/….
    – user4574
    Feb 19, 2023 at 5:48
  • Apparently if it's an Eagle Feather, you can even get a year in jail... kwch.com/content/news/…
    – user4574
    Feb 19, 2023 at 5:50
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    I can’t tell for certain whether you’re relating the “year in jail” story as a factual event or a hyperbolic example. When you say, “Well, that doesn't happen”, what is it that doesn’t happen? Being jailed for a year for such a crime? Feb 19, 2023 at 5:58
  • I wondered the same thing as Tim Feb 19, 2023 at 14:15

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