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.
Wheeler v. Lee, 2015 BCSC 1432 arose out of a single car accident. The plaintiff was the passenger. The defendant was the driver. The defendant’s vehicle was towing a trailer. “The accident occurred when the trailer began to sway or oscillate. It broke free from the trailer hitch and caused the [vehicle] to strike the concrete barrier.” (para. 5)
The issue was liability only. The defendant had loaded the trailer in such a way that there was too much weight at the back of the trailer, contrary to vehicle specifications. According to the expert evidence, the result was to probably reduce the weight carried by the vehicle’s trailer hitch (attached to the rear of the vehicle) below the recommended range. That, in turn, decreased the “critical speed” for this vehicle and trailer combination. The critical speed is a speed at which “increasing sway can occur”. (para. 12, 14-15, para. 20). The expert evidence was:
“Sway is an increasing severe back and forth swinging of a trailer and can be initiated by a gust of wind if the tow vehicle and trailer are travelling near a certain speed.” (para. 12)
“Every tow vehicle and trailer combination will have a speed at which increasing sway can occur. This speed is called the critical speed. A safe tow vehicle and trailer combination will have a critical speed that is high enough to be essentially unattainable. However, there are a number of factors that can reduce the critical speed to a typical highway speed. When travelling at this critical speed, a gust of wind can start sway which will increase in severity.” (para. 14)
“Reducing the amount of weight carried by the trailer hitch decreases the critical speed. The cargo box attached to the rear of the trailer would have reduced the weight carried by the trailer hitch.” (para. 15)
After the sway began, the driver was not able to regain control. There is no suggestion in the reasons that the driver was speeding. There is no suggestion that the driver was negligent in the manner in which he attempted to regain control. The The plaintiff’s allegation was that the driver was negligent in loading the trailer so that the weight on the hitch was below specifications and that “one of the causes of the accident was the fact that there was weight on the trailer hitch that was below the manufacturer’s specifications.” (para. 21)
The trial judge’s summary of the defence position was:
“ The defendant’s position is that there is no evidence the defendant’s response as a driver was in any way inadequate. There is no suggestion that the hitch and braking systems for the trailer and towing vehicle were in any way inadequate. It is the defendant’s submission that it is equally reasonable to infer negligence or no negligence. The plaintiff loses since it is he who bears the legal burden on the issue: … [authority omitted]”
The trial judge set out the (plaintiff’s) expert’s explanation (which the trial judge accepted) of the relationship between the manner in which the trailer had been loaded and the accident at paras. 19-20.
“ He said that the specifications for new travel trailers by this manufacturer suggest hitch weights between 9% and 13% of the trailer weight. The weight carried by the hitch was probably below this recommended range.
 From that he concluded that the presence of the cargo box at the rear of the trailer was probably was “probably” a contributing cause of the trailer sway that led to the accident. He reached these conclusions:
Based on the assumptions and analysis described in this report, I have arrived in the following conclusions:
• the drawbar fractured as a result of the sway and subsequent roll over and did not cause the loss of control.
• the truck and trailer had a number of features (dual trailer axles, load levelers and an anti-sway damper) that, if adjusted properly, would reduce the tendency of the trailer to sway.
• without the cargo box, the trailer hitch was carrying about 12% of the trailer weight.
• with the empty cargo box at the rear of the trailer the hitch was probably carrying about 10% of the trailer weight.
• adding 200 pounds of cargo to the cargo box would probably drop the weight carried by the hitch to a level below the recommended range.
• adding the cargo box to the rear of the trailer decreased the critical speed of the tow vehicle and trailer combination.
• the presence of the cargo box at the rear of the trailer was probably a contributing cause of the trailer sway that led to the accident.”
The trial judge held the defendant was negligent in loading the trailer so that the weight on the vehicle hitch was below the vehicle specifications. “The defendant ought to have known that altering the trailer in this manner would compromise the way in which the trailer operated and in particular, could cause trailer sway.” (para. 29) “The evidence establishes that the defendant’s decision to affix a large box to the rear of the accident and to load it with equipment decreased the tongue weight of the trailer and reduced the ratio to an unsafe extent. The defendant ought reasonably to have known that.” (para. 34)
Trial judge then turned to causation, to decide whether that negligence was a cause of the accident applying the but-for test. On what he wrote, the trial judge did not ask explicitly ask himself whether the negligence in the manner of loading the vehicle was, on the evidence, a necessary cause, or necessarily a part of the cause, or any equivalent phrase. Instead, he asked himself whether the negligence was “one of the causes leading to the accident”. (See paras. 31-32) The trial judge wrote, in para. 31:
 The defendant’s negligence need only be one of the causes leading to the accident: Dhillon v. Jaffer, 2014 BCCA 215 (CanLII) at para. 27:
 …[T]he defendant is not excused from liability merely because other “causal factors” for which he or she is not responsible also helped produce the harm. It is sufficient if the defendant’s negligence was a cause of the harm.”
(Emphasis in original.)
After another quotation from a pre-Clements decision, some of which is now capable of misleading the unwary – at least because of the arguable use of the “materially contributed” phrase in the context of an alternative to but-for method of proving factual causation on the balance of probability – the trial judge set out his conclusion on factual causation. The trial judge did not explicitly write, though he easily could have, in some fashion, that the negligence was a necessary cause of the accident, or was necessary for the occurrence of the accident, etc. Instead, he wrote: ” I am satisfied that the defendant’s negligence was a contributing cause of the accident.” (para. 33)
Did the trial judge mean necessary? I believe he probably did.
The first reasons is the “helped to produce the harm” phrase from Dhillon. That phrase, applied to the facts of Wheeler, implies necessity because, on the reasons, there was no evidence of an alternate, sufficient, explanation of the cause of the sway which did not involve the defendant’s negligence.
For example, although the trial judge stated “[t]he wind may well have been a factor in causing the mishap” (para. 28), the reasons do not mention any evidence that the wind involved was sufficient to have caused the swaying to occur even if the defendant’s manner of loading the trailer had complied with specifications (not been negligent). As such, it seems that the question the judge had to decide to determine the factual causation issue was, simply put: was the negligent manner in which trailer was loaded necessary for the occurrence of the swaying that led to the accident. He had expert evidence that seems to have meant that it probably was.
Another reason is a sentence forming part of the passage he quoted, in para. 32 from para. 17 of Athey. That sentence is: “As long as a defendant is part of the cause of an injury, the defendant is liable, even though his act alone was not enough to create the injury.” (Emphasis in original.) That sentence, in the factual causation context, sets out, clearly enough, the requirement that the negligence need only be a necessary part of the “cause” of the injury to be a legal factual cause of the injury. And, if we are to go back to Athey, Athey emphasizes the necessity requirement, in the but-for context, at para. 41.2:
“If it was necessary to have both the accidents and the pre-existing back condition for the herniation to occur, then causation is proven, since the herniation would not have occurred but for the accidents. Even if the accidents played a minor role, the defendant would be fully liable because the accidents were still a necessary contributing cause”.
(Emphasis in original.)
As such, the better conclusion is that the trial judge probably meant “necessary”, so that the phrase in para. 33 should be understood to mean: “I am satisfied that the defendant’s negligence was a necessary contributing cause of the accident.” Still, it would be better if the trial judge had so so. Then, had he used “necessary” he would not have needed “contributing”. After all, what does “contributing” add to the concept of “cause”? There can’t be a non-contributing cause in the legal factual causation sense. If something is a “cause” of something else, the former contributes to the occurrence of the latter. That’s what cause means.
Also, I believe the expert meant “necessary” because, according to the trial reasons, his conclusion was “the presence of the cargo box at the rear of the trailer was probably a contributing cause of the trailer sway that led to the accident”. The expert would not, I believe, have stated his conclusion that way if it were his opinion that the wind, by itself, was sufficient to have been a cause of the sway even if the trailer had been loaded in such a way that the weight carried by the hitch was within specifications.
However, again, I believe I understand that because I understand the necessity requirement.