Does a plaintiff have to establish that negligence was the sole cause of injury?

In Athey v. Leonati (1996), 1996 CanLII 183 (SCC), [1996] 3 S.C.R. 458; Major, J. at page 239 (D.L.R.) stated: The respondent's position is where a loss is created by tortious and non-tortious causes, it is possible to apportion the loss according to the degree of causation. This is contrary to well-established principles. It has long been established that a defendant is liable for any injuries caused or contributed to by his or her negligence. If the defendant's conduct is found to be a cause of the injury, the presence of other non-tortious contributing causes does not reduce the extent of the defendants liability. . . . It is not now necessary, nor has it ever been, for the plaintiff to establish that the defendants negligence was the sole cause of injury... and at page 243 ... The essential purpose and most basic principle of tort law is that the plaintiff must be placed in the position he or she would have been in absent the defendants negligence (the "original position"). However, the plaintiff is not to be placed in a position better than his or her original one. It is therefore necessary not only to determine the plaintiff's position after the tort but also to assess what the "original position" would have been. If it is the difference between these two positions the "original position" and the "injured position", which is the plaintiffs loss.