What does it meaning of ignorance of the law excuses no one?

"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 presumptionthat 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.