Ignorantia legis neminem excusat is a Latin maxim meaning “ignorance of the law does not excuse” or “ignorance of the law excuses no one.” Awareness of law is a general legal requirement. A person who is unaware of a law cannot escape liability merely because of the unawareness of that law. Lack of knowledge about a legal requirement or prohibition is never an excuse to a criminal charge. In English law, the rule is that ignorance of law is not an excuse. The general rule is that ignorance of fact can be an excuse as far as it negatives mens rea or fault, whereas, ignorance of law cannot be an excuse.
This maxim is often shortened to ignorantia juris. This maxim is also termed as ignorantia juris non excusat, ignorantia legis non excusat, ignorantia juris haud excusat. Originally, this maxim was formulated when the list of crimes represented current morality, but now there are many crimes as a result of administrative or social regulation. The American legal system has recognized certain exceptions to the rule of ignorantia juris, especially in Lambert v. California and Cheek v. United States.
The law imputes knowledge of all laws to all individuals within its jurisdiction. The rationale behind the maxim is that if ignorance of law was recognized as an excuse, individuals charged with criminal offenses or subject to a civil lawsuit would merely claim unawareness of law in question to escape liability even if s/he exactly knows what the law in question is. Therefore, the rule assumes that the law in question was properly published, distributed or printed in an official gazette, and made available to the public over the internet or printed in volumes for sale to public at affordable prices.