What is the test for determining whether a defendant is liable for damages in a motor vehicle accident?

The role that damages plays is to place the plaintiff, as much as possible, in his original position. It is not the obligation of the defendant to put the plaintiff in a better condition than he was in. As noted in Athey v. Leonati, 1996 CanLII 183 (SCC), [1996] 3 S.C.R. 458 at 473-474, 140 D.L.R. (4th) 235, per Mr. Justice Major: The defendant is liable for the injuries caused, even if they are extreme, but need not compensate the plaintiff for any debilitating effects of the pre-existing condition which the plaintiff would have experienced anyway. The defendant is liable for the additional damage but not for the pre-existing damage. ... Likewise, if there is a measurable risk that the pre-existing condition would have detrimentally affected the plaintiff in the future, regardless of the defendant's negligence, then this can be taken into account in reducing the overall award. ... This is consistent with the general rule that the plaintiff must be returned to the position he would have been in, with all of its attendant risks and shortcomings, and not a better position.