Does a plaintiff have to establish that negligence was not the sole cause of injury?

Major J. stated in Athey v. Leonati, 1996 CanLII 183 (SCC), [1996] 3 S.C.R. 458 (S.C.C.) at para. 17: It is not now necessary, nor has it ever been, for the plaintiff to establish that the defendant's negligence was the sole cause of the injury. There will frequently be a myriad of other background events which were necessary preconditions to the injury occurring … As long as a defendant is part of the cause of the injury, the defendant is liable, even though his act alone was not enough to create 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.