What is the test for determining whether a defendant's negligence contributed to a plaintiff's injuries?

The primary test for causation asks: but-for a defendant’s negligence, would the plaintiff have suffered the injury? The “but-for” test recognizes that negligent conduct is only compensable when there is a substantial connection between the injury and the defendant’s conduct: Clements v. Clements, 2012 SCC 32. Causation must be established on a balance of probabilities before damages are assessed: Blackwater v. Plint, 2005 SCC 58 at para. 78.

The most basic principle of tort law is that the plaintiff must be placed in the same position he would have been but for the defendant’s negligence. Tortfeasors must take their victims as they find them, even if the plaintiff’s injuries are more severe than they would be for a normal person. However, a defendant need not compensate a plaintiff for any debilitating effects of a pre-existing condition that the plaintiff would have experienced anyway: Athey, at paras. 32–35. A defendant is responsible for psychiatric injury that is consequential to the physical injury he caused, even if it was unforeseeable: Hussack v. Chilliwack School District No. 33, 2011 BCCA 258 at para. 74.