ABSTRACT
There are many situations in which a claim arises from the receipt of a benefit-for example, a claim
to reverse a mistaken or unauthorised transfer of money or property, or a claim for payment for
services conferred by mistake or in an emergency, or a claim for the surrender up of wrongfully
acquired benefits, or a claim for payment for the unauthorised use of property.2 Some such claims
historically formed a miscellaneous body of claims lying outside the recognised categories of the
common law-contract and tort-and were thought not to be capable of explanation in terms of
these categories. According to the theory of unjust enrichment,3 these various claims are based on
the principle of unjust enrichment.4 This is understood to be a principle equivalent to those that
underlie the traditional categories of law, for example the contractual principle that agreements
should be enforced, or the tortious principle that there is a duty to take reasonable precautions to
avoid causing harm to others.5 These underlying principles provide a guide to the development of
the law in a way that makes it more coherent, bringing together ostensibly different claims that
should be treated in the same way because they are based on the same principle, and eliminating
anomalies and unnecessary obscurity and complexity. The principle of unjust enrichment, it is
thought, promotes the development of a systematic and coherent body of rules analogous to those
of contract and tort, governing claims that arise from the receipt of a benefit.6 It is conceded that
the nature of the principle of unjust enrichment may be controversial and its scope imprecise. But
in this, it is argued, it is no different from the principles underlying contract or tort. These are by
no means uncontroversial, and sometimes controversy is reflected in arguments over the scope and
content of the law, but there is nevertheless sufficient clarity and consensus to provide a workable
principle and a recognised category.