Ignorantia juris non excusat which is also known as ignorantia legis neminem excusat is a legal maxim 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.
This principle is generally applicable to criminal cases. The objective of this maxim is that if ignorance is considered an excuse, a person charged with criminal offenses or a subject of a civil lawsuit would merely claim that she/he is unaware of the law in question to avoid liability.
It may be noted that, ignorance of a municipal law is not allowed to excuse anyone who is of the age of discretion from the penalty for the breach of it, for every such person is bound to know the law of the land regulating his conduct, and she/he is presumed so to know (People v. Klock, 55 Misc. 46 N.Y. County Ct. 1907).
While considering Mens Rea in Statutory Offences, at least one thing is crystal clear, namely, that it does not involve proof that the accused knew that he was committing a criminal offence. It has always been accepted as an axiomatic principle that ignorance of the law is no excuse. Were the position otherwise it is obvious that the legislature's handiwork could be flouted indiscriminately, an offender taking care to insure that he did not make himself cognizant with the law.
However, the maxim ignorantia juris non excusat has no relevance to the case of a man seeking to recover back money paid by him in misapprehension of his legal rights. Thus, we can say that, said legal maxim is in regard to the public, that ignorance cannot be pleaded in excuse of crimes, but that’s not the scenario in civil cases.