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 hallmark of an O. Henry story is its surprise ending.
Because something that’ll occur on May 5, 2015, in Vancouver B.C., may begin to eliminate the problem, I’ll return to a point I’ve made a number of times, on this site, over the past 3 years.
In Clements v. Clements,  2 SCR 181, 2012 SCC 32 (see para. 8) and then in Ediger v. Johnston,  2 SCR 98, 2013 SCC 18 (see para. 28), the S.C.C. stated expressly that the causal relationship between negligence and injury described by the but-for test is one of “necessity” established on the balance of probability. Notwithstanding that, it is still very common – too common – to find statements in reasons for judgment of British Columbia Supreme Court judges which statements, taken at face value, assert a different meaning even when one or both of Clements and Ediger are cited. (Sometimes neither are. Something else, older, is. Really. That’s a different problem.)
I’ve also written that, in at least some of the reported decisions, one can’t tell – or at least I can’t tell – from the text of the reasons what meaning of “but-for” the trial judge applied in deciding that the required causal relationship existed. Perhaps the trial judge did apply the necessity meaning. Perhaps the evidence required that conclusion even if the judge didn’t decide the causation issue that way. However, I’ve suggested that one can’t tell from the reasons. If I’m right, that’s not, all things considered, a “good thing”, even if does create the possibility of more work for lawyers.
I had decided, honouring the “if one can’t say anything good … ” mantra, to stop complaining, at least on this site, about that tendency in reported BCSC reasons. However, I’ve decided that it’s worth mentioning the accurate summary in a very recent Supreme Court of Ontario decision and two more of the erroneous (in my view) summaries in BCSC decisions.
For those not inclined to clink on links as they read, the something that’ll occur is the Continuing Legal Education Society of British Columbia’s programme Causation in Tort III.
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