What is the test for establishing but for causation in a motor vehicle accident case?

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 in Wilsher when he referred to a "robust and pragmatic approach to the ... facts" (p. 569). [Emphasis in Clements] . . .