Today’s decision in Clements v. Clements, 2012 SCC 32 deals with the “but for” test.
On its own, proof by an injured plaintiff that a defendant was negligent does not make that defendant liable for the loss. As a general rule, a plaintiff cannot succeed unless she shows as a matter of fact that she would not have suffered the loss “but for” the negligent act or acts of the defendant. Exceptionally, however, a plaintiff may be able to recover on the basis of material contribution to risk of injury, without showing factual “but for” causation. Elimination of proof of causation as an element of negligence is a radical step that goes against the fundamental principle that a defendant in an action in negligence is a wrongdoer only in respect of the damage which he actually causes to the plaintiff. Therefore, recourse to a material contribution to risk approach is justified only where it is required by fairness and conforms to the principles that ground recovery in tort. The Court holds:
 The foregoing discussion leads me to the following conclusions as to the present state of the law in
(1) As a general rule, a plaintiff cannot succeed unless she shows as a matter of fact that she would not have suffered the loss “but for” the negligent act or acts of the defendant. A trial judge is to take a robust and pragmatic approach to determining if a plaintiff has established that the defendant’s negligence caused her loss. Scientific proof of causation is not required.
(2) Exceptionally, a plaintiff may succeed by showing that the defendant’s conduct materially contributed to risk of the plaintiff’s injury, where (a) the plaintiff has established that her loss would not have occurred “but for” the negligence of two or more tortfeasors, each possibly in fact responsible for the loss; and (b) the plaintiff, through no fault of her own, is unable to show that any one of the possible tortfeasors in fact was the necessary or “but for” cause of her injury, because each can point to one another as the possible “but for” cause of the injury, defeating a finding of causation on a balance of probabilities against anyone.