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We often hear the poplular phrase "ignorance of the law is no excuse", yet mens rea is an essential element of many crimes and torts.

Putting aside morally unambiguous acts such as assault, murder and rape, or things that pose an obvious danger to others, given the vast volume of case law and regulations it is impossible for any individual to know all the laws that apply in a given situation.

If the accused genuinely does not know what law applies to the action they are accused of, how would the prosecution establish mens rea? i.e. How are these seemingly opposed legal priciples reconciled in practice?

Reference - (Google's summary of what it says is found in Oxford languages) Mens Rea: The intention or knowledge of wrongdoing that constitutes part of a crime, as opposed to the action or conduct of the accused.

(empahsis is mine)

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  • possible duplicate - law.stackexchange.com/questions/87982/… Commented Nov 7 at 20:03
  • Neither killing, not hurting, nor having sex with somebody without their informed consent are unambiguously immoral across cultures and eras. The idea that these are to be universally morally rejected is a very modern western phenomenon; until 40 years ago, parents routinely hit their children, and the U.S. government still kills people, with or without trial, and allows child marriage. Commented 2 days ago
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    You might like to read about the mistake of law defense, which does provide some narrow instances where a misunderstanding of the law can be a defense. Commented 5 hours ago

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The mens rea element of an offence almost never requires that the accused know what they're doing is a crime. And that is why ignorance of the law is no excuse.

As Glanville Williams writes in Criminal Law: The General Part, 2nd ed. (1961), p. 30:

a man may have mens rea although he is ignorant of the law. ... the mens rea of English law does not necessarily connote an intention to engage in moral wrongdoing. ... In this respect the phrase mens rea is somewhat misleading.

And in footnote 3, he observes that the modern view of mens rea is simply "an intention [or recklessness] to do the act which is made penal by statute."

The modern view is that mens rea is simply whatever fault element is required by the law to make out an offence. See Williams, p. 31. Mens rea is:

the mental element necessary for the particular crime, and this mental element may be either intention to do the immediate act or being about the consequence or (in some crimes) recklessness as to such act or consequence.

See Rex v. Crowe [1941] 4 DLR 82 (N.S.C.A.):

Perhaps it may be defined most precisely as the mental element necessary to convict for any crime. ... The truth is that the maxim about 'mens rea' means no more than that the definition of all or nearly all crimes contains not only an outward and visible element, but a mental element, varying according to the different nature of different crimes ... Hence the only means of arriving at a full comprehension of the expression 'mens rea' is by a detailed examination of the definitions of particular crimes, and therefore the expression itself is unmeaning.

See also Black's Law Dictionary, 10th ed. (mens rea):

The state of mind that the prosecution, to secure a conviction, must prove that a defendant had when committing a crime

I wouldn't say the definition you quote from Oxford (I personally can't find that source) is wrong, but it is imprecise and clearly open to incorrect readings.

The mens rea often only requires that the person:

  • knowingly did a thing, or
  • did a thing with a particular intention, or
  • did a thing recklessly (i.e. with awareness of the risk of a particular outcome, yet proceeding anyway)

For other offences, like dangerous driving, the fault element is negligence rather than a state of mind.

E.g. it is enough that a person know that they're taking a thing that isn't theirs. That makes out the offence of theft. They don't need to know that theft is an offence.

E.g. it is enough that a person intended to kill a person, and intentionally did the act that killed the person. That makes out the offence of murder. The accused does not need to know that murder is an offence.

E.g. it is enough to knowingly leave an ice fishing hole uncovered and unattended. That makes out an offence under s. 263(3) of the Criminal Code (if someone gets hurt). One does not need to know that this is an offence.

E.g. it is enough to knowingly harass a goose. That makes out an offence under the Migratory Birds Convention Act and its regulations. One does not need to know that this is an offence.

Only for some very specific fraud or obstruction-related offences does the mens rea require an intent to conceal a particular crime that you're aware of.

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  • Comments have been Law Chat. Comments continuing discussion may be removed. Commented 2 days ago
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You don't need to reconcile them

The two doctrines of ignorantia legis neminem excusat (Ignorance of the law excuses nobody('s acts)) and mens rea (guilty mind) are wholly separate:

  • Ignoratia legis neminem excusat basically boils down to that no matter if you know the law, the law is what the law is. You are barred from even bringing a defense that amounts to "but I did not know that X was illegal!"
  • The actus reus is the guilty (prohibited) act. It is the physical side of an event. It's usually rather black and white. Either an event happened or did not. For a lot of things, that alone is sufficient - that's called strict liability.
  • The mens rea is the intent to do the act, not the intent to commit a crime. It's often a factor that is explicitly listed in specific laws and is not a finding that the actor violated what he thought was the law or moral compass knowingly.

Most traffic laws are strict liability: No matter what you think, if you are speeding or cross a red traffic signal, you are guilty of it, and the law says how you are to be punished. You don't need to have had any state of mind: if the state can prove you sped or had a traffic light violation, you're guilty.

For a non-strict liability example, look at theft. The state needs to proove not only that the person took the item, but that they had an intention to deprive the item from the owner. Without the intent to deprive the owner of the item, even if the actus reus is uncontested, the crime's requirements are not met. You need both actus reus and mens rea.

Or to cite Breyer's explanation in Delling v Idaho for the fine line between having a mens rea or none in a case of insanity:

Case One: The defendant, due to insanity, believes that the victim is a wolf. He shoots and kills the victim.

Case Two: The defendant, due to insanity, believes that a wolf, a supernatural figure, has ordered him to kill the victim.

In Case One, the defendant does not know he has killed a human being, and his insanity negates a mental element necessary to commit the crime. In Case Two, the defendant has intentionally killed a victim whom he knows is a human being; he possesses the necessary mens rea.

In both cases the defendant is unable, due to insanity, to appreciate the true quality of his act, and therefore unable to perceive that it is wrong. But … the defendant in Case One could defend the charge by arguing that he lacked the mens rea, whereas the defendant in Case Two would not be able to raise a defense based on his mental illness.

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  • Regarding speeding: you can be guilty of this even if you never saw the speed limit sign, right? And never even could have, because you entered the road between signs. Commented 2 days ago
  • @barmer Speeding is purely fact based, and knowledge of the sign or intent to go fester than the limit are irrelevant. All that matters is the sign being there AMD you being faster than posted Commented 2 days ago
  • @Barmar: That scenario would seem to come down to the legal definition of a "posted" limit, and whether a sign not on the traveled stretch of road is sufficient. Or it might come down to whether the driver is also exceeding the default limit that applies to that type of road where no other limit is posted. Commented 5 hours ago
  • @Barmar: AFAIK, the way this often works is that the law simply makes it a violation to drive faster than is reasonable and prudent; and if the road has been posted (say) 40 MPH, that's treated as prima facie evidence that 40 MPH is the maximum reasonable and prudent speed for that road. So if you're ticketed for going 50, it's not a defense to say you didn't see the sign; your defense would have to show that 50 MPH actually was reasonable and prudent for that road under the conditions -- basically, that you know better than the traffic engineers who set the limit. Commented 5 hours ago
  • I've long been surprised that highways don't have speed limit signs shortly after every entrance ramp. @BenVoigt Commented 5 hours ago

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