What is the principle barring relief for benefits conferred in performance of an illegal contract?

The principle barring relief for benefits conferred in performance of an illegal contract was first stated in Holman v. Johnson (1775), 98 E.R. 1120, where Lord Mansfield stated at 1121: The objection, that a contract is immoral or illegal as between plaintiff and defendant, sounds at all times very ill in the mouth of the defendant. It is not for his sake, however, that the objection is ever allowed; but it is founded in general principles of policy, which the defendant has the advantage of, contrary to the real justice, as between him and the plaintiff, by accident, if I may say so. The principle of public policy is this…No court will lend its aid to a man who founds his cause of action upon an immoral or an illegal act [emphasis added]. On the facts in Holman v. Johnson, Lord Mansfield concluded that the plaintiff had not committed a crime and was entitled to recovery.