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

The courts have established the “but for” test for causation and its application, according to the following principles: The general but not conclusive test for causation is the “but for” test which requires the plaintiff to show that the injury would not have occurred but for the negligence of the defendant… The “but for” test is unworkable in some circumstances so the courts have recognized that causation is established where the negligence of the defendants “materially contributed to the occurrence of the injury… Athey v. Leonati [1997] W.W.R. 97 (S.C.C.). A contributing factor is material if it falls outside the de minimis range… Athey, supra at page 102.

In Snell v. Farrell, 1990 CanLII 70 (SCC), [1990] 2 S.C.R. 311 (S.C.C.) the court stated that the “but for” causation test was not to be applied too rigidly and that causation need not be determined with scientific precision. The causation test is “essentially a practical question of fact which can best be answered by ordinary common sense” (page 328 per Sopinka J.)