What is the "but for" test for proving causation in a motor vehicle accident case?

The “but for” test with respect to causation was set out in the decision in Clements v. Clements, 2012 SCC 32, [2012] 2 S.C.R. 181 at para 8: The test for showing causation is the “but for” test. The plaintiff must show on a balance of probabilities that “but for” the defendant’s negligent act, the injury would not have occurred. Inherent in the phrase “but for” is the requirement that the defendant’s negligence was necessary to bring about the injury ― in other words that the injury would not have occurred without the defendant’s negligence. This is a factual inquiry. If the plaintiff does not establish this on a balance of probabilities, having regard to all the evidence, her action against the defendant fails.