What is the test for establishing causation in a personal injury case?

Turning to the legal principles pertinent to such findings of fact, the leading authority is Athey v. Leonati, 1996 CanLII 183 (SCC), [1996] 3 S.C.R. 458. There, Major J., delivering the judgment of the court, undertook a comprehensive review of the law relating to causation in personal injury cases. In discussion of general principles (at paras. 13-20), he noted that causation is established where the plaintiff proves to the civil standard of a balance of probabilities that the defendant caused or contributed to the injury. Causation may be established either on the "but for" test or by showing that the defendant's negligence "materially contributed" to the occurrence of the injury. It is not necessary to establish that the defendant's negligence was the sole cause of the injury. The defendant whose negligence was a cause of the harm is not excused from liability merely because other (non-tortious) causes helped produce the harm.