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.^[3] 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.^[3] Another case involving the defense of factual impossibility is Commonwealth v. Johnson,^[4] in which a psychic healer was charged and convicted of fraud, despite the fact that a fictitious name was used to catch him. In United States v. Thomas^[5][1] 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 fulfil all the required in elements in a common law or statutory crime. The underlying rationale is that attempting to do what is not a crime is not attempting to commit a crime.^[6] One example of legal impossibility is a person who, thinking that Country 1 has banned the importation of lace from Country 2, attempts to smuggle some "banned" lace into Country 1. The actor believed that her act was a crime, and even fully intended to commit a crime. However, Country 1 does not, in fact, ban lace from Country 2. The traditional approach to understanding the legal impossibility defense is that the mistake (about the content of the law of Country 1) insulates the actor from a conviction for the crime of attempted smuggling. The legal impossibility may be thought of as reflecting that the actor had not satisfied the actus reus of the crime (because they had not actually brought a banned substance into the country). To put it another way, merely trying to commit a crime is insufficient to constitute a criminal attempt; for criminal liability to attach, the actor must be attempting to engage in behaviour that is actually criminal.

Legal impossibility can be distinguished from factual impossibility, which is not generally a defense at common law. Factual impossibility involves an error as to factual reality (the state of the world) that causes the actor to fail to commit a criminal offence when, if the circumstances were as the actor believed, the offence would have been committed. Legal impossibility involves an error as to a legal reality (the state of the law).

However, it is not always easy to identify whether an actor made a legal and factual mistake. In State v. Guffey (1953), the defendant shot a stuffed deer, thinking it was alive and was convicted for attempt to kill a protected animal out of season. In a highly debated reversal, an appellate judge threw out the conviction on the basis of legal impossibility, concluding that it is no crime to shoot a stuffed deer out of season.^[1][3]

• People v. Dlugash
• DPP v Armstrong

• Desmond O'Connor and Paul A. Fairall. "Impossibility". Criminal Defences. Third Edition. Butterworths. 1996. Chapter 7. Pages 117 to 148.
• Jonathan Burchell and John Milton. "Impossibility". Principles of Criminal Law. Second Edition. Juta & Co. 1997. Chapter 16. Page 175 to 177.
• E M Burchell and P M A Hunt. South African Criminal Law and Procedure. Third Edition, by J M Burchell. Juta & Co. 1997. Volume 1. Chapter 10. Page 105 et seq.
• R A Duff. "Impossible Attempts". Criminal Attempts. 1997. Chapter 3. Pages 76 to 115.
• J S Strahorn, "The Effect of Impossibility on Criminal Attempts" (1930) 78 University of Pennsylvania Law Review 962
• Jerome B Elkind, "Impossibility in Criminal Attempts: A Theorist's Headache" (1968) 54 Virginia Law Review 20
• Hellmut A Erwing, "Impossibility as a Defense to Criminal Attempt" in "Notes" (1963) 17 Southwestern Law Journal 461
• John J Yeager, "Effect of Impossibility on Criminal Attempt" (1943) 31 Kentucky Law Journal 270
• Kayla Barkase and David Macallister, "Impossibility in the Law of Criminal Attempt: A Comparison of Canada, Australia and New Zealand" (2014) 14 Oxford University Commwealth Law Journal 153
• David D Friedman, "Impossibility, Subjective Probability, and Punishment for Attempts" (1991) 20 The Journal of Legal Studies 179
• C L Ryan and G P Scanlan, "Attempted Impossibility - Dead or Alive" (1983) 80 The Law Society's Gazette 1902 (27 July 1983)
• "Impossibility and inchoate crimes - Another hook in a red herring" in "Case and Comment" [1993] New Zealand Law Journal 426 Google
• "Impossibility and inchoate offences". Simester and Sullivan’s Criminal Law: Theory and Doctrine. Section 9.5. Pages 392 to 399.
• Nicola Monaghan. "Impossibility". Criminal Law Directions. Fourth Edition. Oxford University Press. 2016. Chapter 15.5.3. Pages 426 and 427.
• Blackstone's Criminal Practice 2012. Pages 82, 99, 103 and 107.
• R S Clark, "The Defence of Impossibility and Offences of Strict Liability" (1968 to 1969) 11 Criminal Law Quarterly 154
• "Immmigration Offence - Defence of Impossibility" (1983) 9 New Zealand Recent Law 82 (April 1983)