What is the test for finding factual causation in a motor vehicle accident?

Once factual causation is found, liability may still be limited in the assessment of damages. I note the well-established principle of tort law that the defendant need not place the plaintiff in a better position than his original position and should not compensate the plaintiff for damage he would have suffered in any event, otherwise known as the crumbling skull rule: Blackwater v. Plint, 2005 SCC 58 at paras. 78 - 81.