An Introduction to the Law of Restitution was a landmark in private law. More clearly than any preceding work, it unpacked the ambiguity inherent in the notion of “the claimant's expense by delineating two forms of “unjust enrichment. (1) The autonomous action in unjust enrichment involves a subtractive expense. The defendant acquires a benefit from the claimant in circumstances that the law regards as reversible. The response is always restitution. The defendant must give the enrichment, or its value, back to the claimant. (2) Unjust enrichment by wrongdoing, in contrast, is concerned with a normative or wrongful expense. The defendant acquires a benefit, usually from a third party, as a result of breaching an obligation owed to the claimant (e.g. trespass to land). Although the standard response to a civil wrong is compensation for the claimant's loss, a court exceptionally may compel the defendant to give up, or disgorge, his ill-gotten gain.

THE issue addressed in this article pertains to the proper measure of relief to which Pam is entitled in an action in unjust enrichment. There is no doubting her ability to establish the constituent elements of that action. Dave was enriched by the receipt of an incontrovertible benefit because he realised a financial gain from the services rendered. That enrichment was derived at Pam's expense (at least in part) because she paid for the repairs. And Dave's enrichment was unjust because Pam's intention in rendering the benefit was vitiated by her mistaken belief that the car was hers. However, it is unclear whether she should be awarded £1000 (reflecting Dave's gain) or £250 (reflecting her loss). The specific question that arises is whether the second element of the principle of unjust enrichment (“at the plaintiff's expense”) merely determines Pam's standing to sue or whether it also limits her measure of relief. It is suggested, contrary to a growing body of opinion, that the latter alternative is preferable.