“The law of unjust enrichment is something of a lost child in every legal system. In a wide range of situations, the law requires that a defendant who has been enriched at the expense of a plaintiff make restitution to that plaintiff, either by returning the very substance of the enrichment, or, more often, by repaying its monetary value. But only if the enrichment is unjust, or unjustified: a gift, for example, is a justified enrichment. This generic description of the scope of the subject can hardly give an inkling of the range of situations in which it plays a role. Some examples include the payment of money by mistake, as when a debtor pays more than they actually owe; improvements to another person’s property, whether or not caused by a mistake regarding ownership; the payment of another’s debt; and the work done by a partner, perhaps over many years, in a cohabitational relationship. This wide range of operation leads us immediately to see one of the most striking examples of diversity among modern legal systems in a field of basic private law …”
Smith, Lionel, ‘Unjust enrichment’, McGill Law Journal, September 2021, volume 66 issue 1, pp 165-168.