What is the test for determining whether a defendant is responsible for a plaintiff’s injuries?

The basic principle of tort law is that the defendant must put the plaintiff back in the position the plaintiff would have been in had the defendant’s tortious act not occurred. The test for causation is the “but for” test. To assess whether a defendant caused an injury, the trial judge asks if, without the defendant’s tortious act, the injury would have resulted. If the answer is “yes,” the defendant is not liable for the injury or the losses flowing from it. If the answer is “no,” the defendant is liable to the plaintiff for the whole of the losses flowing from the injury. The losses “flowing” from an injury are those losses that the plaintiff proves, on a balance of probabilities, would not have occurred “but for” the defendant’s negligent act. The defendant “takes the victim” as the defendant finds her, and is therefore liable even though a plaintiff’s losses are more dramatic than they would be for the average person. On the other hand, the defendant need not compensate the plaintiff for any debilitating effects of a pre-existing condition which the plaintiff would have experienced in any event. If there is a measurable risk that the pre-existing condition would have detrimentally affected the plaintiff in the future, regardless of the defendant’s negligence, then this can be taken into account in reducing the overall award. The general rule is that the plaintiff must be returned to the position she would have been in, with all of its attendant risks and shortcomings, but for the defendant’s negligence, and not a better position. See: Athey v. Leonati, 1996 CanLII 183 (SCC), [1996] 3 S.C.R. 458 at paras. 17, 19-20, 24 and 34-35