What is the test for determining whether a defendant is liable for causing a plaintiff’s injury?

Where cause in fact is established for a given health issue, it is necessary to assess cause in law. This is an assessment of damages issue, as opposed to an assessment of injury. Where there are both tortious and non-tortious causes of an injury, the defendant is liable for the portion of the damages caused by the defendant’s tortious act and 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”: Blackwater v. Plint, 2005 SCC 58, citing Athey.

This is also where the term “crumbling skull” comes into play. A plaintiff who would have suffered the loss had the accident not occurred had a “crumbling skull” and although the accident may have played a role in her injuries, it may not contribute to her loss. But if it contributed to her loss, then she is entitled to be compensated, but only for the portion of the loss caused by the defendants: Gordon v. Ahn, 2017 BCCA 221 at paras. 33 and 36.