What is the test for determining whether a defendant is responsible for an injury?

The seminal decision on the central issue of causation in this case is Athey v. Leonati (1996), 1996 CanLII 183 (SCC), 140 D.L.R. (4th) 235 (S.C.C.). The general principles enunciated in Athey at 238-239 are: (a) Causation is established where the plaintiff proves to the civil standard on a balance of probabilities that the defendant caused or contributed to the injury; (b) The general, but not conclusive, test for causation is the “but for” test, which requires the plaintiff to show that the injury would not have occurred but for the negligence of the defendant; (c) The “but for” test is unworkable in some circumstances, so the courts have recognized that causation is established where the defendant’s negligence “materially contributed” to the occurrence of the injury; (d) Causation need not be determined by scientific precision; and (e) It is not necessary for the plaintiff to establish that the defendant’s negligence was the sole cause of the injury. 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.