It is not enough at law for a Plaintiff to prove that a defendant is negligent, as a Plaintiff must also prove that the negligence of a Defendant actually caused the injury.
The general, but not conclusive, test for causation is the “but for” test. A Plaintiff must show, on a balance of probabilities, that “but for” (without) the negligence of the Defendant, then the Plaintiff’s injury would not have happened.
The “but for” test is not workable in some situations, so courts also establish causation where the negligence of the Defendant “materially contributes” to the occurrence of the injury i.e. beyond the de minimus range.
The “but for” test also recognizes that compensation for the negligence of another should only be made where there is a substantial connection between the injury and the conduct of the Defendant. This ensures that the Defendant will not be held liable for a Plaintiff’s injuries when such injuries may be due to factors that are not connected to the Defendant.
There is no requirement for a Plaintiff to show that the negligence of a Defendant was the sole cause of the injury to the Plaintiff. So long as the defendant was only part of the cause of an injury, then that Defendant will be liable, even if his or her act alone was not enough to cause the injury to the Plaintiff. The Defendant is responsible for all injuries substantially connected to, or caused by, the negligence of the Defendant.
In Perry v. Vargas, the Plaintiff was injured in a motor vehicle accident, and brought an ICBC claim for injuries. There were several other non-tortious (not related to the car accident) causes of the Plaintiff’s injuries as well. The Court would eventually rule that, based on a preponderance of probabilities, it was not prepared to find that the motor vehicle accident was the cause of the Plaintiff going from working full-time, to not being able to work full-time. The Court also provides useful commentary on the relevant principles of law with respect to the issue of causation.
 In Resurfice Corp. v. Hanke, 2007 SCC 7 (CanLII), 2007 SCC 7 at paras. 21 to 23, 2007 SCC 7 (CanLII),  1 S.C.R. 333, the Supreme Court of Canada confirmed that the basic test for determining causation remains the “but for” test, and that the test applies to multi-cause injuries. However, the Court developed the “but for” test by stating a “substantial connection” between the injury and the defendant’s conduct is necessary in cases where the defendant’s act was not the sole cause of the plaintiff’s injury.
 The developments in the “but for” test in Hanke were succinctly summarized by Madam Justice Neilson in Farrant v. Laktin, 2011 BCCA 336 (CanLII), 2011 BCCA 336 at paras. 9 to 11, 338 D.L.R. (4th) 527
 The general test for causation, established in Athey v. Leonati ……. If there are other potential non-tortious causes, such as the plaintiff’s spinal degeneration in this case, the defendant will still be found liable if the plaintiff can prove the accident caused or materially contributed to the disabling pain, beyond the de minimus range.
 In Hanke v. Resurfice Corp., 2007 SCC 7 (CanLII), 2007 SCC 7,  1 S.C.R. 333, … the Court replaced the Athey definition of “material contribution” to the plaintiff’s injury with the concept of “a substantial connection” between the injury and the defendant’s conduct. These developments were usefully summarized by Mr. Justice Smith, writing for the majority, in Sam v. Wilson, 2007 BCCA 622 (CanLII), 2007 BCCA 622 at para. 109:
… the “material contribution test” applies as an exception to the “but for” test of causation when it is impossible for the plaintiff to prove that the defendant’s negligent conduct caused the plaintiff’s injury using the “but for” test, where it is clear that the defendant breached a duty of care owed the plaintiff thereby exposing the plaintiff to an unreasonable risk of injury, and where the plaintiff’s injury falls within the ambit of the risk. …
 Thus, in applying the “but for” test, the trial judge was required to consider not just whether the defendant’s conduct was the sole cause of the plaintiff’s disabling pain, but also whether the plaintiff had established a substantial connection between the accident and that pain, beyond the de minimus level.
 Accordingly, to satisfy the “but for” test, a plaintiff must show that the defendant caused his or her injury by proving on a balance of probabilities that the defendant’s conduct was the sole cause of his or her injury, or that there was a substantial connection between the defendant’s actions and the injury, beyond the de minimus level.
 In short, I am not satisfied on a balance of probabilities that, but for the 2006 Accident, Ms. Perry would be having these ongoing symptoms. Nor am I satisfied that the debilitating headaches she experienced shortly before going on medical leave are related to the 2006 Accident. In my view it has not been proven, on a balance of probabilities, that her change from being capable of full-time work to being incapable of full-time work, occurring two years after the 2006 Accident, is related to any of the injuries she suffered in the 2006 Accident.