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Ignorantia legis neminem excusat.
Ignorance of the law excuses no one.

Also given as Ignorantia iuris (juris) non excusat, "Ignorance of the law does not excuse". An even broader expression is Ignorantia non excusat, "Ignorance does not excuse"; which goes beyond the realm of law, providing us with the criticism of some unfortunate one who says, "But I didn't know . . . ."