Ignorantia juris non excusat

The maxim means that ignorance of the law excuses no one. Everyone is presumed to know the law of the land, and a person who breaks it cannot defend himself by saying he did not know it existed or misunderstood it. Without the rule, any accused could plead ignorance, and the law would be unworkable.

Indian criminal law applies the maxim through the general exceptions. Section 14 of the BNS excuses an act done by a person who, by reason of a mistake of fact and not by reason of a mistake of law, in good faith believes himself bound by law to do it. Section 17 gives the same protection to a person who, by a mistake of fact and not of law, in good faith believes himself justified by law. These sections were earlier Sections 76 and 79 of the IPC. The words "not by reason of a mistake of law" give statutory form to the maxim, and the companion maxim ignorantia facti excusat explains why a mistake of fact can excuse.

Contract law takes the same position. Under Section 21 of the Indian Contract Act, 1872, a contract is not voidable because it was caused by a mistake as to any law in force in India, but a mistake as to a law not in force in India has the same effect as a mistake of fact. Foreign law is thus treated as a question of fact.

The rule presupposes that the law has been made known. In Harla v. State of Rajasthan (1951), the Supreme Court held that a measure passed by a resolution of a council of ministers but never promulgated or published was not operative law, so a conviction under it could not stand. Examiners test the meaning, the mistake of law and mistake of fact contrast in Sections 14 and 17 of the BNS, and Section 21 of the Contract Act.