In this legal causation personal injury case, (Clements v. Clements, 2012 SCC 32) the Supreme Court of Canada has done its best to clarify for lawyers and claimants when the “but for” causation test can be replaced by the material contribution to risk test in car accident injury cases stating:
“ The foregoing discussion leads me to the following conclusions as to the present state of the law in Canada:
(1) As a general rule, a plaintiff [injury claimant] cannot succeed unless she shows as a matter of fact that she would not have suffered the loss “but for” the negligent act or acts of the defendant. A trial judge is to take a robust and pragmatic approach to determining if a plaintiff has established that the defendant’s negligence caused her loss. Scientific proof of causation is not required.
(2) Exceptionally, a plaintiff may succeed by showing that the defendant’s conduct materially contributed to risk of the plaintiff’s injury, where (a) the plaintiff has established that her loss would not have occurred “but for” the negligence of two or more tortfeasors, each possibly in fact responsible for the loss; and (b) the plaintiff, through no fault of her own, is unable to show that any one of the possible tortfeasors in fact was the necessary or “but for” cause of her injury, because each can point to one another as the possible “but for” cause of the injury, defeating a finding of causation on a balance of probabilities against anyone. ”
The injury claimant’s husband was driving a motorcycle and the claimant was the passenger near Prince George, British Columbia. The bike was overloaded and the husband was speeding. Unknown to the injury claimant a nail had punctured the bike’s rear tire. The husband crossed the centre line to pass a vehicle and the nail fell out, the tire deflated, and the bike began to wobble. The husband was unable to bring the bike under control and it crashed, throwing the claimant off. The claimant suffered a severe traumatic brain injury.
The husband’s negligence in driving an overloaded bike too fast was not disputed and the only issue was whether his negligence caused his wife’s injury. The trial judge found that the husband’s negligence in fact contributed to the claimant’s injury and applied the “material contribution” test.
The British Columbia Court of Appeal set aside the judgment against the husband on the basis that “but for” causation had not been proved and the material contribution test did not apply(2010 BCCA 581).
The Supreme Court of Canada concluded in this decision that the material contribution test was not applicable and returned the matter to the trial judge to be dealt with on the correct basis of “but for” causation. Chief Justice McLachlin for the majority discounted the legal analysis of the BC Court of Appeal stating at paragraph 45, “It was unnecessary, in my view, to hang the analysis on “circular causation”, and “dependency causation”, which may complicate the matter rather than simplify it.” As McLachlin C.J. stated starting at paragraph 9:
The “but for” causation test must be applied in a robust common sense fashion. There is no need for scientific evidence of the precise contribution the defendant’s negligence made to the injury. See Wilsher v. Essex Area Health Authority,  A.C. 1074, at p. 1090, per Lord Bridge; Snell v. Farrell,  2 S.C.R. 311. A common sense inference of “but for” causation from proof of negligence usually flows without difficulty. Evidence connecting the breach of duty to the injury suffered may permit the judge, depending on the circumstances, to infer that the defendant’s negligence probably caused the loss. See Snell and Athey v. Leonati,  3 S.C.R. 458. See also the discussion on this issue by the Australian courts: Betts v. Whittingslowe,  HCA 31, 71 C.L.R. 637, at p. 649; Bennett v. Minister of Community Welfare,  HCA 27, 176 C.L.R. 408, at pp. 415-16; Flounders v. Millar,  NSWCA 238, 49 M.V.R. 53; Roads and Traffic Authority v. Royal,  HCA 19, 245 A.L.R. 653, at paras. 137-44.
Where “but for” causation is established by inference only, it is open to the defendant to argue or call evidence that the accident would have happened without the defendant’s negligence, i.e. that the negligence was not a necessary cause of the injury, which was, in any event, inevitable. As Sopinka J. put it in Snell, at p. 330:
The legal or ultimate burden remains with the plaintiff, but in the absence of evidence to the contrary adduced by the defendant, an inference of causation may be drawn although positive or scientific proof of causation has not been adduced. If some evidence to the contrary is adduced by the defendant, the trial judge is entitled to take account of Lord Mansfield’s famous precept [that “all evidence is to be weighed according to the proof which it was in the power of one side to have produced, and in the power of the other to have contradicted” (Blatch v. Archer (1774), 1 Cowp. 63, 98 E.R. 969, at p. 970)]. This is, I believe, what Lord Bridge had in mind inWilsher when he referred to a “robust and pragmatic approach to the … facts” (p. 569). [Emphasis added.]
This case emphasizes the importance of judges making legal causation decisions despite the lack of scientific evidence. In this vein, McLachlin C.J. goes on to state:
 “Scientific impossibility”, relied on by the trial judge in this case, is merely a variant of factual impossibility and attracts the same objections. In many cases of causal uncertainty, it is conceivable that with better scientific evidence, causation could be clarified. Scientific uncertainty was referred to in Resurfice in the course of explaining the difficulties that have arisen in the cases. However, this should not be read as ousting the “but for” test for causation in negligence actions. The law of negligence has never required scientific proof of causation; to repeat yet again, common sense inferences from the facts may suffice. If scientific evidence of causation is not required, as Snell makes plain, it is difficult to see how its absence can be raised as a basis for ousting the usual “but for” test. …
 The trial judge made two errors.
 The first error was to insist on scientific reconstruction evidence as a necessary condition of finding “but for” causation. The trial judge stated, at para. 66 that . . . the plaintiff through no fault of her own is unable to prove that “but for” the defendant’s breaches, she would not have been injured. This is because after the fact, it is not possible through accident reconstruction modeling to determine at what combination of lower speed and lesser weight recovery from the weave instability would have been practicable.
 As discussed above, the cases consistently hold that scientific precision is not necessary to a conclusion that “but for” causation is established on a balance of probabilities. It follows that the trial judge erred in insisting on scientific precision in the evidence as a condition of finding “but for” causation.
LeBel and Rothstein JJ. dissented stating, “ I have read the Chief Justice’s reasons. I agree with the substance of her analysis of the law of causation and the nature of the “but for” test. But, in my respectful opinion, there is no basis in fact and law for ordering a new trial. I would uphold the judgment of the Court of Appeal and dismiss the appeal.”
In my view LeBel JJ.’s comments at para 61 with respect to sending this case back for a new trial is much more practical with respect to a fair outcome for the injury claimant, “I wonder whether the order for a new trial itself represents sound judicial policy. I am not arguing that this Court lacks jurisdiction to issue this order and that the order is therefore illegal. But, on policy grounds related to the administration of justice and the conduct of civil appeals, this Court and courts of appeal should be mindful of the need for finality and efficiency in the civil litigation process. Where it is appropriate to do so, an attempt should be made to resolve the issues and thereby avoid sending the matter back for a new trial, which might itself trigger a new round of appeals.”
It could certainly be argued that the Supreme Court of Canada had more than enough facts to find that the “but for” test had been met and the husband was fully liable for the claimant’s injuries. Personal injury claimants should obtain proper legal advice about how this decision may affect their claim. Posted by Personal Injury Lawyer Mr. Renn A. Holness B.A. LL.B.