Impossibility defense

An impossibility defense is a criminal law argument raised when a defendant is charged with a criminal attempt that failed only because completing the crime was factually or legally impossible, rather than because the defendant chose not to go through with it. It is distinct from a mistake of fact defense, which applies to specific intent crimes such as larceny and concerns the defendant's understanding of the circumstances rather than whether the crime itself could have been completed. Courts distinguish between factual impossibility, where the defendant did not realize that some circumstance made completing the crime impossible, such as attempting to pick an empty pocket, and legal impossibility, where the act the defendant intended was not actually a crime at all. Under the common law, factual impossibility rarely succeeds as a defense, since the defendant still intended and attempted a criminal act, while legal impossibility has historically been treated more favorably because no crime existed to attempt. The doctrine belongs to criminal law and should not be confused with the similarly named impossibility doctrine in contract law, which concerns whether a contractual obligation can still be performed rather than whether an attempted crime could be completed.