What is the test for apportioning damages between tortious causes and non-tortious causes?

In Athey v. Leonati et al. (1996), 1996 CanLII 183 (SCC), 203 N.R. 36; 81 B.C.A.C. 243; 132 W.A.C. 243 (S.C.C.), Major, J., for the court, reviewed some basic principles in tort law. Among those are that "the law does not excuse a defendant from liability merely because other causal factors for which he is not responsible also helped produce the harm." The law does not permit apportionment between tortious causes and nontortious causes as it "would be contrary to established principles and the essential purpose of tort law, which is to restore the plaintiff to the position he or she would have enjoyed but for the negligence of the defendant." Of course, in the case of multiple tortious causes, "apportionment between tortious causes is expressly permitted by provincial negligence statutes and is consistent with the general principles of tort law. The plaintiff is still fully compensated and is placed in the position he or she would have been in but for the negligence of the defendants. Each defendant remains fully liable to the plaintiff for the injury, since each was a cause of the injury."