A legal excuse or defense to an action for the breach of a contract; less frequently, a defense to a criminal charge of an attempted crime, such as attempted ROBBERY or murder.
Historically, a person who entered a contract was bound to perform according to his or her promised duties, regardless of whether it became impossible to do so. Thus, early U.S. courts did not recognize the defense of impossibility of performance. Courts noted that if the parties to a contract had desired to take into account any events that may develop after they reached an agreement, then they should have accounted for such contingencies in the contract.
As contract law developed over the twentieth century—and in response to increasing commercial activities—courts began to recognize impossibility as a valid defense to an action for breach of a contract. This defense did not normally apply if one party found it unexpectedly difficult or expensive to perform according to the contract; rather, it applied only when the basis or subject matter of the contract was destroyed or no longer existed. In addition, the defense of impossibility became available only if objective impossibility existed. Objective impossibility occurred when the contractual obligation could not actually be performed. Objective impossibility is often referred to by the statement "The thing cannot be done." For example, if a musician promised to play a concert at a specific concert hall but the concert hall subsequently burned down, it would be impossible to perform according to the contractual agreement and the musician would be excused from performing at that particular venue. Subjective impossibility exists when only one of the parties to a contract subjectively believes that she or he cannot complete the required performance. For example, if a musician believed that he had not practiced sufficiently to perform a successful concert, this belief would not excuse the musician from performing the concert. The statement "I cannot do it" frequently refers to the state of mind present in a case involving subjective impossibility.
Modern U.S. law uses the term impracticability synonymously with the term impossibility, primarily because some things may not be absolutely impossible to perform but are nevertheless impracticable to complete. Thus, the general rule is that a thing may be impossible to perform when it would not be practicable to perform. A contractual obligation is impracticable "when it can only be done at an excessive and unreasonable cost" (Transatlantic Financing Corp. v. United States, 363 F.2d 312 [D.C. Cir. 1966]).
When a party raises the defense of impracticability, courts generally determine three things: first, whether something unexpected occurred after the parties entered the contract; second, whether the parties had assumed that this thing would not occur; and third, that the unexpected occurrence made performance of the contract impracticable. Some widely recognized occurrences that would normally provide a defense of impracticability are the death or illness of one of the necessary parties, the unfore-seeable destruction of the subject matter of the contract (perhaps by an "act of God"), or a supervening illegality.
Impossibility has been used as a defense to charges of attempted crimes. Historically, courts recognized that a party could not be convicted of criminal attempt if the actual crime was legally impossible to accomplish. For example, if a person was accused of attempting to receive stolen property but the property was not actually stolen, the defense of legal impossibility could arise. Legal impossibility is distinguished from factual impossibility, where facts unknown to the person attempting to commit a crime render the crime factually impossible to complete. For example, if a pickpocket attempts to steal a wallet but no wallet is present, factual impossibility may exist. Courts generally have recognized legal impossibility as a defense to a criminal attempt, but not factual impossibility. They reasoned that since a person attempting to commit a crime had formed the required intent to commit the crime, it was irrelevant that the crime was factually impossible to complete.
Impossibility as a defense to a criminal attempt has largely been rejected by modern U.S. statutes and courts. The Model Penal Code—which many states have adopted since its introduction in 1962—expressly rejects impossibility as a defense to the charge of criminal attempt (§ 5.01 ).