Impossibility of performance is a defense used in contract law to excuse the performance of one of the parties. In order for the defense to succeed, the party claiming it must not have contributed to the conditions making performance imposssible. Impossibility must be due to unforeseen and uncontrollable circumstances, such as death, destruction of the subject matter, or failure of the means of deliver.
Courts generally have recognized legal impossibility as a defense to a criminal attempt, but not factual impossibility. Factual impossibility exists where facts unknown to the person attempting to commit a crime prevent the crime from being commited. For example, a jewel thief may be unaware that the jewel safe has been emptied. The Model Penal Code, adopted by many states, expressly rejects impossibility as a defense to the charge of criminal attempt. The courts reason that it it irrelevant that the crime is factually impossible to complete, as long as criminal intent exists.
