Most criminal law systems in developed states exclude mistake of law as a defense, because allowing defendants to invoke their own ignorance of the law would breach the public policy represented by the Latin maxim: "ignorantia legis neminem excusat". But someone operating under a mistake of fact will not generally be liable, because, although the defendant has committed the "actus reus" of the offense, the defendant may honestly believe in a set of facts that would prevent him or her from forming the requisite "mens rea" required to constitute the crime.