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

A plaintiff does not have to establish that a defendant’s negligence was the sole cause of the injury. McLachlin C.J. explained this in Blackwater v. Plint ¶78: 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.