(a) A person’s ignorance or mistake as to a matter of either fact or law, except as provided in Section 4-3(c) above, is a defense if it negatives the existence of the mental state which the statute prescribes with respect to an element of the offense.
     (b) A person’s reasonable belief that his conduct does not constitute an offense is a defense if:

Terms Used In Illinois Compiled Statutes 720 ILCS 5/4-8

  • Appellate: About appeals; an appellate court has the power to review the judgement of another lower court or tribunal.
  • State: when applied to different parts of the United States, may be construed to include the District of Columbia and the several territories, and the words "United States" may be construed to include the said district and territories. See Illinois Compiled Statutes 5 ILCS 70/1.14
  • Statute: A law passed by a legislature.
  • United States: may be construed to include the said district and territories. See Illinois Compiled Statutes 5 ILCS 70/1.14

         (1) the offense is defined by an administrative
    
regulation or order which is not known to him and has not been published or otherwise made reasonably available to him, and he could not have acquired such knowledge by the exercise of due diligence pursuant to facts known to him; or
        (2) he acts in reliance upon a statute which later is
    
determined to be invalid; or
        (3) he acts in reliance upon an order or opinion of
    
an Illinois Appellate or Supreme Court, or a United States appellate court later overruled or reversed; or
        (4) he acts in reliance upon an official
    
interpretation of the statute, regulation or order defining the offense, made by a public officer or agency legally authorized to interpret such statute.
    (c) Although a person's ignorance or mistake of fact or law, or reasonable belief, described in this Section 4-8 is a defense to the offense charged, he may be convicted of an included offense of which he would be guilty if the fact or law were as he believed it to be.
     (d) A defense based upon this Section 4-8 is an affirmative defense.