The impossibility of carrying out a contract, which occurs, for example, when it relates to subject matter that does not exist. The event making fulfilment impossible may arise either before or after the contract is made. In the former case (e.g. if X agrees to sell Y a horse that, unknown to either, is already dead) the contract is void for mistake. In the latter case (e.g. if the horse dies between contract and performance) the contract will be discharged under the doctrine of frustration of contract.