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.
[3] 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.
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).
By the beginning of the 16th century, lawyers recognised a distinct species of action on the case known as assumpsit, which had become the typical phrase in the pleadings.
[8] For a plaintiff, assumpsit was the more desirable course: the defendant would not be able to elect to wage his law as he would in debt sur contract.
It is important to note that where assumpsit was brought in lieu of debt, the plaintiff's action was for a liquidated sum.
In contrast, where a plaintiff brought special assumpsit, the action was for an unliquidated sum assessed by the civil jury.
Examples of the common counts include: By the 18th and 19th centuries, the action of assumpsit was used to enforce both contractual and quasi-contractual claims.
The recognition in Slade's Case that the law would import or imply a promise to pay the debt paved the way for other implications.
The reason for this is historical: where there was no deed, the correct action was assumpsit for nonfeasance; in the latter, in debt sur obligation.
[13] In the law of unjust enrichment, reference is still made to actions for money had and received and quantum meruit.