The doctrine of impossibility or impossibility of performance or impossibility of performance of contract is a doctrine in contract law. In contract law, 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 example, if Ebenezer contracts to pay Erasmus £100 to paint his house on October 1, but the house burns to the ground before the end of September, Ebenezer is excused from his duty to pay Erasmus the £100, and Erasmus is excused from his duty to paint Ebenezer's house; however, Erasmus may still be able to sue under the theory of unjust enrichment for the value of any benefit he conferred on Ebenezer before his house burned down. 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. Sometimes it is impossible to perform a contract as a result of war. As to impossibility of performance of contract during war, see Arnold Duncan McNair, "War-Time Impossibility of Performance of Contract", Essays and Lectures Upon Some Legal Effects of War, Cambridge, at the University Press, 1920, Chapter 5, p 78; Henry Campbell, "Impossibility of Performance", The Law of War and Contract, Humphrey Milford, Oxford University Press, 1918, chapter 5, p 263 et seq Google; K V R Townsend, "Impossibility of Performance of Contracts due to War-Time Governmental Interference" (1941) 8 Current Legal Thought 150; "Impossiblity of Performance of Contracts due to War-Time Regulations" (1918 to 1919) 32 Harvard Law Review 789; Weber, The Effect of War on Contracts, 2nd Ed, 1946, p 445 et seq; MacKinnon, Effect of War on Contract: Being an Attempted Analysis of the Doctrine of Discharge of Contract by Impossibility of Performance: With a Résumé of the Principal Cases decided in the English Courts during the Present War, 1917 Google; Trotter, The Law of Contract during War, 1914, p 57, 58 and 59 et seq Google; Trotter, The Law of Contract during and after War, 3rd Ed, 1919, p 118 et seq; Trotter, Supplement to the Law of Contract during War, 1915, pp 76, 77 and 78 et seq Google; and Coleman Philipson, "Where new circumstances would make lawful fulfilment impossible", The Effect of War on Contracts, 1909, p 78 Google.

Australia

Frustration usually involves impossibility of performance. As to whether the non-existence of the subject matter of a contract constitutes (initial) impossibility of performance, see McRae v Commonwealth Disposals Commission. Cf. Res extincta.

England and Wales

A contract may be frustrated by impossibility of performance. In 1997, Downes said that impossibility and impracticability were separate in England and Wales, and that impracticability was not usually found to result in frustration. The English case that established the doctrine of impossibility at common law is Taylor v Caldwell.

New Zealand

Wilkins and Davies Construction Co Ltd v Geraldine Corporation [1958] NZLR 985 is relevant.

South Africa

A contract may be ended by supervening impossibility of performance. As to supervening impossibility of performance in South Africa, see William Arthur Ramsden, Supervening Impossibility of Performance in the South African Law of Contract, Juta & Co, 1985, ISBN 0-7021-1629-7; William Arthur Ramsden, Supervening Impossibility of Performance in the South African Law of Contract, University of the Witwatersrand, 1983; William Arthur Ramsden, "Some Historical Aspects of Supervening Impossibility of Performance of Contract" (1975) 38 Tydskrif vir Hedendaagse Romeins-Hollandse Reg 153, 284 and 370; William Arthur Ramsden, "Temporary Supervening Impossibility of Performance" (1977) 94 South African Law Journal 162; Jacques du Plessis, "Supervening Impossibility of Performance – A Conceptual Conundrum and Restatement of Principles" (2022) 33 Stellenbosch Law Review 605Sabinet; A J Kerr, "The Effect of Leases on Supervening Impossibility of Performance" (1977) 94 South African Law Journal 389; "Termination by supervening impossibility of performance", The Law of South Africa, Butterworths, 2006, vol 19, para 249 at p 170; Louis F van Huyssteen and Catherine J Maxwell, Contract Law in South Africa, 2021, paras 386 to 398; "Rendering performance impossible", Wille's Principles of South African Law, 8th Ed, Juta, 1991, p 514; J T R Gibson, "Supervening Impossibility of Performance", South African Mercantile and Company Law, 2nd Ed, Juta & Company, 1966, p 95; A J Kerr, "Breach, Supervening Impossibility and Risk" (1978) 95 South African Law Journal 16; and Martin Brassey, "The Effect of Supervening Impossibility of Performance on a Contract of Employment" [1990] Acta Juridica 22.

United States

At common law, for the defence of "impossibility" 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, in the United States it is beginning to be recognized that "impossibility" under this doctrine can also exist when the contemplated performance can be done but only at an excessive and unreasonable cost, i.e., commercial impracticability. On the other hand, some US sources see "impossibility" and impracticability as being related but separate defences.