Squishy. That’s been the rap on the law of restitution since before there even was a law of restitution. In Moses v. Macferlan, Lord Mansfield stated that whenever the defendant is "under an obligation, from the ties of natural justice, to refund" the money, the law allows an "action [for the money received], founded in the equity of the plaintiff’s case." At the outset of the project that has now resulted in the Restatement (Third) of Restitution and Unjust Enrichment, Reporter Andrew Kull confronted this problem head on. At the ALI’s initial consideration of the project in 2000, he noted the perceived "amorphous" nature of the subject, remarking that many people have felt that "a purported legal principle that necessarily appeals to a shared sense of what equity and good conscience require is untrustworthy." Has the effort to make restitution more determinate succeeded, or is the law of restitution still saddled with a degree of reliance on vague concepts of justice that marks it as distinctly different from its cousins, the laws of tort and contract?
So, is restitution indeterminate? Yes and no. Yes, there is an unavoidable element of indeterminacy in the effort to state the rules of the law of restitution. That, however, is hardly a phenomenon unique to the law of restitution. If restitution is indeterminate, so are contract and tort. The fact that the current Restatement makes such liberal use of phrases like "as necessary to avoid unjust enrichment" is merely a matter of drafting style, attributable in large measure to the fact that this field of law is still relatively poorly understood relative to other fields of law. Courts dealing with restitution claims should be no more willing to explain their decisions merely by vague references to unjust enrichment than they would feel about making similar assertions in cases involving other legal subjects. There is more to the Restatement than the truism in Section 1 that "[a] person who has been unjustly enriched at the expense of another is required to make restitution to the other." There is, however, no use denying that in some areas it is difficult or impossible to state the rules of restitution in simple black-and-white terms.
James S. Rogers. "Indeterminacy and the Law of Restitution." Washington & Lee Law Review 68, no.3 (2011): 1377-1405.