The judges of the Common Pleas, however, a more traditional group, rejected this argument and only accepted cases where an actual promise had been made in addition to the contract.

[5] This, and the conflict between the King's Bench and the Common Pleas as a whole, was problematic; a plaintiff at assizes could not be sure which sort of judge his case would come before, lending uncertainty to the law.

Boyer suggests that, in this environment, the Chief Justice of the King's Bench John Popham deliberately provoked the Common Pleas to resolve the matter, and did so through Slade's Case.

[6] John Slade was a grain merchant, who claimed that Humphrey Morley had agreed to buy a crop of wheat and rye from him, paying £16, and had reneged on the agreement.

[8] Edward Coke was counsel for Slade, arguing that the King's Bench had the power to hear assumpsit actions, along with Laurence Tanfield, while Francis Bacon and John Doddridge represented Morley.

[8] Coke, rather than directly confronting opposing counsel, made a twofold argument; firstly, that the fact that the King's Bench had been allowed to hear assumpsit actions for so long meant that it was acceptable, based on institutional inertia, and second that, on the subject of assumpsit being used for breaches of promise, that the original agreement included an implied promise to make payment.

Eventually, in November 1602, Popham issued a judgment on behalf of the court which stated "Firstly, that every contract executory implies in itself a promise or assumpsit.

Ibbetson considers Slade's Case to be a "watershed" moment, in which the archaic and conservative form of law was overwritten by a modern, more efficient method.

In his Commentaries on the Laws of England, William Blackstone explained that this was the reason why the Statute of Frauds was subsequently passed in 1677: Some agreements indeed, though ever so expressly made, are deemed to be of so important a nature, that they ought not to rest in verbal promise only, which cannot be proved but by the memory (which sometimes will induce the perjury) of witnesses.