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

In these cases, it is also important to keep in mind the distinct principles of causation applicable to (a) liability and (b) the assessment of damages. With respect to causation for liability purposes, the basic principles are found in the seminal case of Athey v. Leonati, 1996 CanLII 183 (SCC), [1996] 3 S.C.R. 458 at paras. 13-20, repeated many times since, and which include: 1. The general, but not necessarily conclusive test for causation is the "but for" test requiring the plaintiff show his injury and loss would not have occurred but for the negligence of the defendant. 2. This causation test must not be applied too rigidly. Causation need not be determined by scientific precision as it is essentially a practical question of fact best answered by ordinary common sense. 3. It is not necessary for the plaintiff to establish that the defendant's negligence was the sole cause of the injury and damage. As long as it is part of the cause of an injury, the defendant is liable. 4. Apportionment does not lie between tortious causes and non-tortious causes of the injury or loss. The law does not excuse the defendant from liability merely because causal factors for which he is not responsible also helped to produce the harm.