A test in tort law linking the tort and the damages (aka causation), which is stated as: but for the defendant's negligence, the plaintiff would not have been injured.
In Clements v Clements, Justice of Canada's Supreme Court used these words to offer a neat summary of the law and the context in which the concept of but for appies in tort law:
"Recovery in negligence presupposes a relationship between the plaintiff and defendant based on the existence of a duty of care — a defendant who is at fault and a plaintiff who has been injured by that fault. If the defendant breaches this duty and thereby causes injury to the plaintiff, the law corrects the deficiency in the relationship by requiring the defendant to compensate the plaintiff for the injury suffered. This basis for recovery, sometimes referred to as corrective justice, assigns liability when the plaintiff and defendant are linked in a correlative relationship of doer and sufferer of the same harm.
"The test for showing causation is the but for test. The plaintiff must show on a balance of probabilities that but for the defendant’s negligent act, the injury would not have occurred.
"Inherent in the phrase but for is the requirement that the defendant’s negligence was necessary to bring about the injury ? in other words that the injury would not have occurred without the defendant’s negligence. This is a factual inquiry. If the plaintiff does not establish this on a balance of probabilities, having regard to all the evidence, her action against the defendant fails.
"The but for causation test must be applied in a robust common sense fashion. There is no need for scientific evidence of the precise contribution the defendant’s negligence made to the injury.
"A common sense inference of but for causation from proof of negligence usually flows without difficulty. Evidence connecting the breach of duty to the injury suffered may permit the judge, depending on the circumstances, to infer that the defendant’s negligence probably caused the loss.
"Where but for causation is established by inference only, it is open to the defendant to argue or call evidence that the accident would have happened without the defendant’s negligence, i.e. that the negligence was not a necessary cause of the injury, which was, in any event, inevitable."
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 suffered.
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,  3 SCR 458:
"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 ( 2 SCR 311), 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 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...."