What is the test for determining whether a defendant is liable for any injuries caused or contributed to by their negligence?

In the case of Athey v. Leonati, 1996 CanLII 183 (SCC), [1996] 3 S.C.R. 458 (S.C.C.), the headnote reads in part as follows: A defendant is liable for any injuries caused or contributed to by his or her negligence. The presence of other non-tortious contributing causes does not reduce the extent of that liability. Loss cannot be apportioned according to the degree of causation where it is created by tortious and non-tortious causes. Causation is established where the plaintiff proves to the civil standard that the defendant caused or contributed to the injury. 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. Where the “but for” test is unworkable, the courts have recognized that causation is established where the defendant’s negligence “materially contributed” to the occurrence of the injury.

Major, J., stated at page 472 of Athey v. Leonati, supra: … 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 defendant’s 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. It is the difference between these positions, the “original position” and the “injured position”, which is the plaintiff’s loss. Damages