On a plain reading of the “but for” test, it would seem that it requires you, the plaintiff, to establish that the defendant’s negligence was the sole cause of your injuries and impairments, or the loss, however, the Supreme Court of Canada, while providing a rational basis for the need to have the aforementioned material contribution test, reminded us that this not a necessary requirement in the case entitled Athey v. Leonati, supra, at paragraphs 17, 19 and 20, as follows:
It is not now necessary, nor has it ever been, for the plaintiff to establish that the defendant’s negligence was the sole cause of the injury. There will frequently be a myriad of other background events which were necessary preconditions to the injury occurring … There is no basis for a reduction of liability because of the existence of other preconditions: defendants remain liable for all injuries caused or contributed to by their negligence.
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: Fleming, supra, at p. 200. It is sufficient if the defendant’s negligence was a cause of the harm: School Division of Assiniboine South, No. 3 v. Greater Winnipeg Gas Co.,  4 W.W.R. 746 (Man. C.A.), at p. 753, aff’d  6 W.W.R. 765 (S.C.C.),  S.C.R. vi; Ken Cooper-Stephenson, Personal Injury Damages in Canada (2nd ed. 1996), at p. 748.
This position is entrenched in our law and there is no reason at present to depart from it. If the law permitted apportionment between tortious causes and non-tortious causes, a plaintiff could recover 100 percent of his or her loss only when the defendant’s negligence was the sole cause of the injuries. Since most events are the result of a complex set of causes, there will frequently be non-tortious causes contributing to the injury. Defendants could frequently and easily identify non-tortious contributing causes, so plaintiffs would rarely receive full compensation even after proving that the defendant caused the injury. This 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.
Therefore, one of the primary guiding principles behind the awarding of compensatory damages in common law negligence claims is captured in the term restitio ad integrum or restitutio in integrum, which is a Latin term that means restoration to original condition, or to restore you to the position you would have enjoyed “but for” the negligence of the defendant. Since this purpose would never be achieved if the law required the defendant’s negligence to be the sole cause of your injuries and impairments, or the loss, it has become entrenched in our law that the defendant could still be held 100% liable even if there are, as is often the case, a myriad of other background events, and a complex set of pre-conditions and non-tortious causes to the damages you sustained. So, if you, as the plaintiff, are able to prove on a balance of probabilities that the defendant’s negligent act materially contributed to your injury and impairment, or loss complained of, in a manner that falls outside of the de minimis range, then you satisfy the onus of proving causation, further to Kersey v. Wellesley Hospital  46 C.C.L.T. 271 (Ont.H.C.J.). As such, there would be no basis for a reduction of liability because of the existence of other preconditions and non-tortious causes, so the defendant would remain liable for all of your injuries and impairments, or the loss caused or contributed to by the defendants negligence, further to Cork v. Kirby MacLean Limited  2 All E.R. 402 (C.A.) and Athey v. Leonati, supra.