Assumpsit

Assumpsit ("he has undertaken", from Latin, assumere), or more fully, the action of assumpsit, was a form of action at common law. The origins of the action can be traced to the 14th century, when litigants seeking justice in the royal courts turned from the writs of covenant and debt to the trespass on the case.

Throughout its long history, the action of assumpsit has been used to enforce what are now referred to as obligations arising in tort, contract, and unjust enrichment. Most significantly, it is out of the law relating to the action of assumpsit that the modern law of contract and of unjust enrichment emerged in the 19th, 20th and 21st centuries.

In the early days of the English common law, agreements were enforced in local courts. Where one wished to enforce an agreement in the royal courts, it was necessary to fit one's claim within a form of action. In the 13th and 14th centuries the forms of action for the enforcement of agreements were covenant, debt, detinue, and account. These were all writs in the praecipe form, meaning that they commanded the defendant to perform an act: for example, to keep a promise; to yield up a sum of money or chattel unjustly withheld; or to render accounts.

These actions were subject to various limitations. For example, by the middle of the 14th century at the latest, it was necessary for a plaintiff in an action of covenant to have a deed. In an action of debt sur contract, a deed was not necessary, but a defendant was able to wage his law and the sum claimed had to be a sum certain fixed at the date of contract. Such rules could easily work hardship. What if a promisor (A) orally agreed to supply grain to a promisee (B), but failed to do so? In such a case, B would be unable to bring a writ of trespass due to the absence of a deed. B would instead bring debt sur contract, bringing a number of transaction witnesses. But what if A elected wager of law and simply hired his eleven oath-helpers?

Litigants began to turn from the praecipe writs of covenant and debt to the ostensurus quare writ of trespass. By the middle of the 14th century the royal courts were recognising that a writ of trespass would lie even without an allegation that the defendant had acted vi et armis contra pacem regis (with force and arms against the King's Peace). This action became known as trespass on the case.

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