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.
The counterfactual analysis aspect
A case worth reading, because it accurately explains how the but-for counterfactual hypothetical (i.e., possible, what-if, etc., world) analysis is to be done. (For those who have been paying attention, that’s the analysis for a test that we’ve been told isn’t metaphysical at all, which is one reason why we’re to use it rather than go adventuring with philosophers into abstract metaphysics, or something like that.)
Lee v Minister for Correctional Services,  ZACC 30, 2013 (1) SACR 213 (CC) (11 December 2012) starting at para. 37.
As a colleague put it, the court’s explanation is “far better than anything our Supreme Court has ever come up with”.
One case that isn’t helpful on the “how” of the counterfactual analysis – even if, for other reasons, the result of the case is correct: I make no comment on that point – is Eli Lilly and Company v. Apotex Inc., 2014 FC 1254 at paras. 20-35. I believe that, after you read Lee, if you read Lee, you’ll understand why the discussion in Eli Lilly isn’t helpful. And why the explanation is wrong, even if the conclusion is correct.
The how much evidence is enough aspect – i.e., the Snell robust and pragmatic inference
Judges, we know, have long memories. They talk amongst themselves. It doesn’t do a lawyer’s clients any good, let alone the lawyer’s reputation, if judges begin with the assumption counsel doesn’t know the applicable law.
With the end approaching of what would have been, once upon a time, the “long vacation” and litigators gearing up for the fall court season, it’s time for a fashion tip reminder.
The reminder is that is is no longer the law in common law Canada that, somehow, the but-for test is to be applied differently in civil cases where expert evidence is adduced on the issue of factual causation, even in those places where it was once fashionable to assert it was; even in those places in common law Canada where some seem to think it is still in fashion to assert it is.