How has the doctrine of ex turpi causa been interpreted?

McLachlin J. (as she then was), for the majority of the court in Hall v. Hebert, 1993 CanLII 141 (SCC), [1993] 2 S.C.R. 159 at para. 7, recognized that ex turpi causa “has its roots in the insistence of the courts that the judicial process not be used for abusive, illegal purposes”. According to her (at para. 12), application of the doctrine in contract is premised on the basis that: “the court will not assist a wrongdoer in profiting from an illegal scheme or act”.

The effect of illegality is procedural rather than substantive; the plaintiff is precluded from enforcing the illegal contract. It is for the defendant to establish that the plaintiff is relying on an illegal contract to prove his case: Wilkinson v. Harwood, 1930 CanLII 11 (SCC), [1931] S.C.R. 141.