What is "ignorantia juris non excusat"

phr. ignorance of the law is no valid excuse.

Ignorantia juris non excusat or ignorantia legis neminem excusat ( Latin for " ignorance of the law excuses not" and "ignorance of law excuses no one" respectively) 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.

European-law countries with a tradition of Roman law may also use an expression from Aristotle translated into Latin: nemo censetur ignorare legem (nobody is thought to be ignorant of the law) or ignorantia iuris nocet (not knowing the law is harmful).