Some of you may consider what follows excessive pedantry because you’ve already realized where I’m going and are horse-fanciers and members of an SPCA-type organization whose purview extends to dead animals (or zombies).
In Clements v. Clements,  2 SCR 181, 2012 SCC 32, the Supreme Court set out the requirements of the but-for test for proof of factual causation in negligence. The Court made it explicitly clear that the test turns on the requirement of “necessity”. The defendant’s negligence must have been necessary for the occurrence of the accident, and ultimately the plaintiff’s injury. Any causal relationship less than necessity will not satisfy the but-for test.
“ 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.”
(Emphasis in original.)
Everything in Clements after paragraph 8, dealing with the but-for test, explains how the test is to be applied; what the judge or jury is to do in order to decide if the evidence permits the valid conclusion that, on the balance of probability, the negligence was “necessary” . The discussion does not lessen, reduce – choose any synonym you want – the “necessity” requirement.
This isn’t just me on a soapbox.
In Ediger v. Johnston,  2 SCR 98, 2013 SCC 18, the Court summarized the Clements message.
 This Court recently summarized the legal test for causation in Clements v. Clements, 2012 SCC 32 (CanLII),  2 S.C.R. 181. Causation is assessed using the “but for” test (Clements, at paras. 8 and 13; Resurfice Corp. v. Hanke, 2007 SCC 7 (CanLII),  1 S.C.R. 333, at paras. 21-22). That is, the plaintiff must show on a balance of probabilities that “but for” the defendant’s negligent act, the injury would not have occurred (Clements, at para. 8). “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” (para. 8 (emphasis deleted)).
After the break, I discuss I recent case in which I think the trial judge, and the plaintiff’s expert, probably meant there was a necessary relationship between the negligence and the accident, but the trial judge did not explicitly quote or paraphrase paragraph 8 of Clements. The expert’s testimony, as set out in the reasons, also does not necessarily contain any of the magic words or an explicit synonym (word or phrase).
The trial judge’s summary of his conclusion on causation was: “I am satisfied that the defendant’s negligence was a contributing cause of the accident.” (para. 33)
Did the trial judge mean: I am satisfied that the defendant’s negligence was necessary for the occurrence of the accident in the sense that, but-for the negligence of the defendant, the accident would not have occurred.