The doctrine of supervening impossibility

The doctrine. A contract that could be performed when it was made becomes void if, afterwards, an unexpected event that the promisor could not prevent makes performance impossible or unlawful; the parties are then discharged from further performance (s. 56). This is subsequent, supervening or post-contractual impossibility.

Cases:

1. Destruction of the subject matter essential to performance: a contract to repaint a particular bus ends if the bus is destroyed in a depot fire before the work begins.
2. Failure of a state of things the parties assumed, including personal incapacity in a contract of a personal nature: a pianist booked for a recital who loses the use of her hand before the date is excused.
3. A change in the law that makes performance unlawful.
4. Declaration of war, where it makes performance impossible or unlawful.

Exceptions (limits). Performance that has merely become harder or costlier is not impossible, so the contract stands; nor does a strike or lock-out, or the failure of only one of several purposes, by itself discharge it.

Distinguish initial impossibility. Where the act was already impossible when the contract was made, and the promisor alone knew, or with reasonable diligence should have known, of it, he must compensate the promisee for loss caused by the non-performance (s. 56, third paragraph). That rule concerns the time of contracting, not a later event.