(B) THE EX TURPI CAUSA PRINCIPLE
All restitutionary claims, regardless of the underlying cause of action, are subject to a defence of illegality. This stems from the fundamental rule, known as ex turpi causa non oritur actio (‘No action can arise from a base cause’ ), meaning that the courts will not assist a claimant to obtain a remedy where the action is founded on illegal conduct. This rule was recognized by Lord Mansfield in Holman v Johnson:
The objection, that a contract is immoral or illegal as between the 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 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 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.
This rule negating liability is explicitly founded on policy rather than justice. As Lord Goff
said in Tinsley v Milligan:
it is a principle of policy, whose application is indiscriminate and so can lead to unfair consequences as between the parties to litigation. Moreover the principle allows no room for the exercise of any discretion by the court in favour of one party or the other.
-  Windeyer J in Smith v Jenkins (1969) 119 CLR 397, 411 considered that ‘causa’ did not refer to ‘cause ofaction’ but referred to illegal or immoral consideration, so the doctrine was only relevant to contractual claims.This was rejected by Dillon LJ in Pitts v Hunt  1 QB 24, 56.
-  (1775) 1 Cowp 341, 343; 98 ER 1120, 1121. See also Muckleston v Brown (1801) 6 Ves Jun 52, 69; 31 ER934, 942 (Lord Eldon LC).
-   1 AC 340, 355. See also Gray v Thames Trains Ltd  UKHL 33,  1 AC 1339,  (LordHoffmann).