On 19 May 1760, Lord Mansfield (left) delivered judgment in the famous case of Moses v Macferlan (1760) 2 Burr 1005, 97 ER 676, [1558-1774] All ER Rep 581,  EngR 713 (19 May 1760) (warning: pdf; an unofficial html is here).
Much of the modern law of restitution has been constructed upon the foundations which he laid down:
If the defendant be under an obligation, from the ties of natural justice; to refund; the law implies a debt, and gives this action, founded in the equity of the plaintiff’s case, as it were upon a contract (“quasi ex contractu,” as the Roman law expresses it).
This species of assumpsit, (“for money had and received to the plaintiffs use,”) lies in numberless instances, for money the defendant has received from a third person; which he claims title to, in opposition to the plaintiff’s right; and which he had, by law, authority to receive from such third person. …
This kind of equitable action, to recover back money, which ought not in justice to be kept, is very beneficial, and therefore much encouraged. It lies only for money which, ex aequo et bono, the defendant ought to refund: it does not lie for money paid by the plaintiff, which is claimed of him as payable in point of honor and honesty, although it could not have been recovered from him by any course of law; as in payment of a debt barred by the Statute of Limitations, or contracted during his infancy, or to the extent of principal and legal interest upon an usurious contract, or, for money fairly lost at play; because in all these cases, the defendant may retain it with a safe conscience, though by positive law be was barred from recovering. But it lies for money paid by mistake; or upon a consideration which happens to fail; or for money got through imposition, (express, or implied;) or extortion; or oppression; or an undue advantage taken of the plaintiff’s situation, contrary to laws made for the protection of persons under those circumstances.
In one word, the gist of this kind of action is, that the defendant, up on the circumstance of the case, is obliged by the ties of natural justice and equity to refund the money.
Two hundred and fifty years later, to the very day, the American Law Institute – a leading independent organization in the United States producing scholarly work to clarify, modernize, and otherwise improve the law – approved a new Restatement of the Law Third, Restitution and Unjust Enrichment. It is the fruits of fourteen years’ work, under the wise leadership of Prof Andrew Kull, Paul M. Siskind Research Scholar and Professor of Law at Boston University School of Law. When published later this year, it will replace the original Restatement of the Law of Restitution promulgated in 1936. On the ALI’s blog, Scott Maravilla recorded the historic moment in this way:
The Boskey Amendment approving the Restatement of the Law (Third) on Restitution and Unjust Enrichment passed unanimously. The historical significance was not lost with references to Lord Mansfield. There was large amounts of well deserved praise for Andrew Kull. John Cameron of Michigan praised Kull ‘s leadership. Boskey called the Restatement
a remarkable piece of work for which we are all grateful to the Reporter.
Kull received a standing ovation from the room for his work. In a lighter moment, as the proceedings were moving to a close, Douglas Laycock called to the attention of the floor that the membership hadn ‘t yet voted. Everyone then voted unanimously.
This is a momentous achievement. It has taken profound legal scholarship, adroit drafting skills, superhuman reserves of energy and patience, and virtuoso leadership. Andrew Kull has accomplished the extraordinary, and I am in awe of his outstanding success, all the more historic for the date on which it occurred.