What is the test for determining causation in a medical malpractice action?

The plaintiff submits that causation has been relaxed in some circumstances permitting an inference of causation to be made where “positive or scientific proof has not been advanced.” In Snell v. Farrell, 1990 CanLII 70 (SCC), [1990] 2 S.C.R. 311, the court observed: 33 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. 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). 34 It is not therefore essential that the medical experts provide a firm opinion supporting the plaintiff's theory of causation. Medical experts ordinarily determine causation in terms of certainties whereas a lesser standard is demanded by the law. V