What is the test for determining whether a tortfeasor is liable for a plaintiff's injuries?

Safety Boss also referred to these comments made by McLachlin C.J. in Blackwater v. Plint, 2005 SCC 58, 3 SCR 3 at paras. 78 and 80: It is important to distinguish between causation as the source of the loss and the rules of damage assessment in tort. The rules of causation consider generally whether “but for” the defendant’s acts, the plaintiff’s damages would have been incurred on a balance of probabilities. Even though there may be several tortious and non-tortious causes of injury, so long as the defendant’s act is a cause of the plaintiff’s damage, the defendant is fully liable for that damage. The rules of damages then consider what the original position of the plaintiff wold have been. The governing principle is that the defendant need not put the plaintiff in a better position than his original position and should not compensate the plaintiff for any damages he would have suffered anyway: [Athey]. ... Where a second wrongful act or contributory negligence of the plaintiff occurs after or along with the first wrongful act, yet another scenario, sometimes called the “crumbling skull” scenario, may arise. Each tortfeasor is entitled to have the consequences of the acts of the other tortfeasor taken into account. The defendant must compensate for the damages it actually caused but need not compensate for the debilitating effects of the other wrongful act that would have occurred anyway. This means that the damages of the tortfeasor may be reduced by reason of other contributing causes: [Athey].