IGNORANTIA JURIS NON EXCUSAT (IGNORANCE OF THE LAW EXCUSES NO ONE)

- is a legal principle holding that a person who is unaware of a law may not escape liability for violating that law merely because he or she was unaware of its content.

Rationale:If ignorance were an excuse, a person charged with criminal offenses or a subject of a civil lawsuit would merely claim that he or she is unaware of the law in question to avoid liability, even if that person really does know what the law in question is. 

“willful blindness cannot become the basis of exculpation” meaning, ignorance of the law or deliberately claiming ignorance is not ground for case dismissal or absolution (acquittal).

"There is a true law, right reason, agreeable to nature, known to all men, constant and eternal, which calls to duty by its precepts, deters from evil by its prohibition. This law cannot be departed from without guilt. Nor is there one law at Rome and another at Athens, one thing now and another afterward; but the same law, unchanging and eternal, binds all races of man and all times."

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"Ignorance of the law is no excuse" is a common phrase in the law. It means that if an individual breaks a law, they cannot defend themselves by claiming that they did not know that there was a law to be broken, or that they were breaking it. 

This topic is often tied in with "mens rea." Mens rea is a concept the law uses to describe a mental element of a crime. Often, a crime is defined as requiring that the individual have both committed the act of the crime (the "actus reus") and intended to commit the crime ("mens rea.") There was a time when criminals would claim that they did not intend to commit the crime, because they did not know that the act which they committed was a crime. 

This is why the law developed the concept that "ignorance of the law is no excuse." It creates an irrebutable presumption that every person knows what the law is, and that no one can claim that they did not have the mens rea because they did not know what the law is.

Mens rea – mental intent to do the crimeActus reus – physical act of the crime

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"Generally, ignorance of the law is no excuse, although there are some exceptions. An exception (is) ... where the charge was one of willful failure to comply with probation and the breach was an allegation of committing a criminal offence with a separate mens rea. A further exception is recognized to the general rule that ignorance of the law is no excuse, for cases of officially induced error."

NOTE: from a source

“Ignorance of the law is no excuse.” was not based on the notion that people could read the entire legal code, and thus could be held responsible for not having done so. It was based on the assumption that most laws were malum in se, that a person of normal moral sense wouldn’t need to read the law to know a particular violation was something they shouldn’t be doing, because the law prohibited moral wrongs.

It’s not so much the proliferation of laws that makes the concept nonsensical, as the proliferation of malum prohibitum laws, of laws outlawing conduct a person of normal moral sense wouldn’t realize were wrong to commit, because the only thing wrong with doing them was that somebody had passed a law.Perhaps the remedy is a government that is more modest about what it’s entitled to enact laws about?