Impossibility

In the law of contracts, impossibility is an excuse for the nonperformance of duties under a contract, based on a change in circumstances (or the discovery of preexisting circumstances), the nonoccurrence of which was an underlying assumption of the contract, that makes performance of the contract literally impossible. For such a defense to be raised, performance must not merely be difficult or unexpectedly costly for one party; there must be no way for it to actually be accomplished.

However, the parties to a contract 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.