Ignorantia facti excusat, ignorantia juris non excusat

Meaning: Ignorance of fact excuses; ignorance of law does not excuse.
Explanation: A mistake about facts may sometimes relieve a person from liability, but not knowing the law itself is generally never accepted as a defense.
Example: Taking someone else’s identical bag by honest mistake at an airport may be excusable, but claiming ignorance of theft laws is not.