Ignorantia juris non excusat

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 one 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).

The rationale of the doctrine is that if ignorance were an excuse, a person charged with criminal offenses or a subject of a civil lawsuit would merely claim that one was unaware of the law in question to avoid liability, even if that person really does know what the law in question is. Thus, the law imputes knowledge of all laws to all persons within the jurisdiction no matter how transiently. Even though it would be impossible, even for someone with substantial legal training, to be aware of every law in operation in every aspect of a state's activities, this is the price paid to ensure that willful blindness cannot become the basis of exculpation. Thus, it is well settled that persons engaged in any undertakings outside what is common for a normal person, such as running a nuclear power plant, will make themselves aware of the laws necessary to engage in that undertaking. If they do not, they cannot complain if they incur liability.

The doctrine assumes that the law in question has been properly promulgated—published and distributed, for example, by being printed in a government gazette, made available over the internet, or printed in volumes available for sale to the public at affordable prices. In the ancient phrase of Gratian, Leges instituuntur cum promulgantur ("Laws are instituted when they are promulgated"). In order that a law obtain the binding force which is proper to a law, it must be applied to the men who have to be ruled by it. Such application is made by their being given notice by promulgation. A law can only bind when it is reasonably possible for those to whom it applies may acquire knowledge of it in order to observe it, even if actual knowledge of the law is absent for a particular individual. A secret law is no law at all.

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Wikipedia