A test in tort law linking the tort and the damages (aka causation), which are stated as: "but for" the defendant's negligence, the plaintiff would not have been injured.
As Justice Cameron wrote for the Newfoundland Court of Appeal in Bow Valley Husky (Bermuda) Ltd. v. Saint John Shipbuilding Ltd., cited at 126 DLR 4th 1 (1995):
"The but for test has been accepted as an appropriate method of deciding causation in negligence cases....
"It asks the question -- would the accident have occurred but for the defendant's negligence? If the answer is that the accident would have occurred even without the defendant's negligence, there is no causation."
When suing for negligence, a plaintiff must show that the actions of the alleged tort feasor (who will be the defendant in the tort action) caused the injury or damages the plaintiff sufferred.
May also be referred to as the sine quo non (without which not) test and in American law, the "but for" test is at times also referred to as "factual causation".
In Athey v Leonati 1996 3 SCR 458 (published at canlii.org/en/ca/scc/doc/1996/1996canlii183/1996canlii183.html):
"Causation is established where the plaintiff proves to the civil standard on a balance of probabilities that the defendant caused or contributed to the injury.
"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."
Also, in an earlier Supreme Court of Canada decision (1990 2 SCR 311, published at canlii.org/en/ca/scc/doc/1990/1990canlii70/1990canlii70.html), Snell v Farrell, dealing with medical liability, the Court summarized the basic plaintiff's burden of proof in a negligence claim:
"... the plaintiff must prove on a balance of probabilities that, but for the tortious conduct of the defendant, the plaintiff would not have sustained the injury complained of."
Where the "but for" test cannot be applied, such as where there are more than one contributing factor, it may be possible to presume causation anyway under the doctrine of a 2007 Supreme Court of Canada case, Resurfice v Hanke 2007 SCC 7 (published at canlii.org/en/ca/scc/doc/2007/2007scc7/2007scc7.html) and which also restarted the but for test adopting these words:
"The basic test for determining causation remains the but for test. This applies to multi-cause injuries. The plaintiff bears the burden of showing that “but for” the negligent act or omission of each defendant, the injury would not have occurred. Having done this, contributory negligence may be apportioned, as permitted by statute.
"This fundamental rule has never been displaced and remains the primary test for causation in negligence actions.
"(T)he 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”. Similarly ... the rules of causation consider generally whether ‘but for’ the defendant’s acts, the plaintiff’s damages would have been incurred on a balance of probabilities.
"The but for test recognizes that compensation for negligent conduct should only be made where a substantial connection between the injury and the defendant’s conduct is present. It ensures that a defendant will not be held liable for the plaintiff’s injuries where they may very well be due to factors unconnected to the defendant and not the fault of anyone...."
REFERENCES
Duhaime, Lloyd, Causation, published at duhaime.org/LegalResources/TortPersonalInjury/tabid/348/articleType/ArticleView/articleId/74/Causation.aspx