What position does the court take in respect of an unlawful contract?

The most oft-quoted statement as to the position the courts take in respect of an unlawful contract is that of Lord Mansfield's in Holraan v. Johnson (1775) 1 Cowp. 341 at 343, "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: ex dolo malo non oritur actio. No court will lend its aid to a man who founds his cause of action upon an immoral or an illegal act. If, from the plaintiff's own stating or otherwise, the cause of action appears to arise ex turpi causa, or the transgression of a positive law of this country, there the court says he has no right to be assisted. It is upon that ground the court goes; not for the sake of the defendant, but because they will not lend their aid to such a plaintiff. So if the plaintiff and defendant were to change sides and the defendant was to bring his action against the plaintiff, the latter would then have the advantage of it; for where both are equally in fault, potior est conditio defendentis."