Impossibility is an excuse for non-performance of duties under a contract, based on a change in circumstances (or the discovery of preexisting circumstances) 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.

For example, if Rachel offers to pay Joey $500 to paint her house on October 1, but the house burns to the ground before the end of September, Rachel is excused from her duty to pay Joey the $500, and he is excused from the duty to paint her house.