THE recovery at law of money paid under mistake affords one of the most striking illustrations of the equitable nature of quasi-contractual obligations, — obligations that are, unfortunately, usually called in our law, contracts implied in law.
Where A intentionally pays money to B, and B intentionally receives it as payment from A, plainly the legal title has passed; and a Court of law, if the money has been paid under a proper case of mistake, compels B to restore to A the money so received, not because the Court does not regard B as the legal owner thereof, but because it is inequitable that he should retain it.
The equitable principle which enables A to recover in this case, as in quasi-contractual obligations generally, is the principle of enrichment: “One shall not be allowed to unjustly enrich himself at the expense of another;” or, as it is usually stated in the common law, “One shall not unjustly profit at the expense of another.”
It is proposed in the present article to deal with the general principles under which one is allowed to recover money paid under mistake of fact, and not with their application in detail to specific facts, except so far as it is necessary to a correct understanding of the principles. And the question will be discussed