Impossibility of performance

Impossibility of performance is a doctrine in contract law, and in that setting the impossibility defense is a defense offered against a claim for nonperformance. Impossibility excuses the nonperformance of contractual duties where a change in circumstances, or the discovery of preexisting circumstances, whose nonoccurrence was an underlying assumption of the contract, makes performance of the contract literally impossible. The standard illustration is a contract to paint a house on a fixed date where the house burns to the ground beforehand: both the payment duty and the painting duty are excused, although the painter may still be able to sue in unjust enrichment for the value of any benefit already conferred. Performance can also become impossible as a result of war. Parties may choose to ignore impossibility by inserting a hell or high water clause, which mandates that payments continue even if completion of the contract becomes physically impossible.