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

The determination of causation in this case is rendered complex by both the pre-existing and subsequent illnesses from which the plaintiff suffered, and by reason of the two previous motor vehicle accidents and the prior store incident, each of which injured the plaintiff. The principles to be applied were articulated in Athey v. Leonati, 1996 CanLII 183 (SCC), [1996] 3 S.C.R. 458, at pp. 466-468. I summarize those principles as follows: (a) causation is established where the plaintiff proves to the civil standard on a balance of probabilities that the defendant caused or contributed to the injury; (b) the “but for” test is the general test for causation. Where this test is unworkable, causation is established where the defendant’s negligence materially contributed to the occurrence of the injury; (c) the defendant’s negligence need not be the sole cause of injury; so long as it is a material cause, the defendant will be liable. There is no apportionment between tortious and non-tortuous causes; (d) the plaintiff is to be restored to the position she would have been in absent the defendant’s negligence. Therefore the defendant need not compensate the plaintiff for the debilitating effects of pre-existing conditions, nor for subsequent events which are completely unrelated to the defendant’s negligence.