An Impossibility defense is a criminal defense occasionally used when a defendant is accused of a criminal attempt that failed only because the crime was factually or legally impossible to commit. Factual impossibility is rarely an adequate defense at common law. In the United States, thirty-seven states have ruled out factual impossibility as a defense to the crime of attempt. This is not to be confused with a 'mistake of fact' defense, which may be a defense to a specific intent crime like larceny.
An impossibility occurs when, at the time of the attempt, the facts make the intended crime impossible to commit although the defendant is unaware of this when the attempt is made. In People v. Lee Kong, 95 Cal. 666, 30 P. 800 (1892), the defendant was found guilty for attempted murder for shooting at a hole in the roof, believing his victim to be there, and indeed, where his victim had been only moments before but was not at the time of the shooting. Another case involving the defense of factual impossibility is Commonwealth v. Johnson, in which a wife intended to put arsenic in her husband's coffee but by mistake added the customary sugar instead. Later she felt repentant and confessed her acts to the police. She was arrested, tried, and convicted of attempted murder. In United States v. Thomas the court held that men who believed they were raping a drunken, unconscious woman were guilty of attempted rape, even though the woman was actually dead at the time sexual intercourse took place.
An act that is considered legally impossible to commit is traditionally considered a valid defense for a person who was being prosecuted for a criminal attempt. An attempt is considered to be a "legal" impossibility when the defendant has completed all of his intended acts, but his acts fail to fulfill all the required in elements of a crime in common law. Mistakes of law have proved a successful defense. An example of a failed attempt of law is a person who shoots at a tree stump, believing that he is committing attempted murder; that person cannot be prosecuted for attempted murder as there is no manifest intent to kill by shooting a stump. The underlying rationale is that attempting to do what is not a crime is not attempting to commit a crime.
However, "legal" and "factual" mistakes are not mutually exclusive. A borderline case is that of a person who shot a stuffed deer, thinking it was alive as was the case in State v. Guffey, (1953) in which a person was originally convicted for attempting to kill a protected animal out of season. In a debatable reversal, an appellate judge threw out the conviction on the basis that it is no crime to shoot a stuffed deer out of season.
- Richard M. Bonnie, Anne M. Coughlin, John C. Jefferies, Jr. & Peter W. Low (1997). Criminal Law. Westbury, NY: The Foundation Press. p. 251. ISBN 1-56662-448-7.
- John Hasnas (2002). "Once More unto the Breach:The Inherent Liberalism of the Criminal Law and Liability for Attempting the Impossible". George Mason University School of Law - Hastings Law Journal. p. 13. Retrieved 2008-01-25.
- George P. Fletcher. Rethinking Criminal Law. Oxford University Press. pp. 149–151. Retrieved 2008-01-25.
- 167 A. 344, 348 (Pa. 1933)
- 13 U.S.C.M.A. 278 (1962)
- "Criminal Law & Criminal Procedure Case Briefs - United States v. Thomas - Court of Military Appeals, 1962". Retrieved 2008-01-25.
- "Attempt -Impossibility Unavailable as a Defense". Oklahoma Jury Instructions. Retrieved 2008-01-25.