This is a discussion of principle and principal.
As ever, it wouldn’t hurt to have a glass or cup of your favourite beverage at your side while you ponder.
In tort, in order for the plaintiff to be entitled to succeed against a defendant, assuming the plaintiff will be able to satisfy all of the other requirements of the cause of action, the court must find that that required causal connection exists between the tortious conduct for which the defendant is responsible and the plaintiff’s losses.
Some of you might wonder, assuming the court concludes that the required causal connection exists, why it matters whether that causal connection, which forms part of basis upon which a defendant is held liable, is to be understood as
1. a finding of factual causation established on the balance of probability – it is more likely than not that the tortious conduct did cause the loss; or
2. a legal fiction under which law deems the relevant facts to amount to a finding of factual causation on the balance of probability as if such a finding had actually been made, regardless of whether they do or do not; or,
3. a conclusion of law based on “policy” that, as the tortious conduct could have been a factual cause, it will be held to be a factual cause, even though the tortious conduct cannot validly be shown to have been a factual cause on a more likely than not basis.
Let’s call each of these alternatives a “doctrinal meaning” of causation and, for short, the “meaning” of causation.
It does not matter to plaintiff(s) or to defendant(s) held liable if the consequences of a finding of liability based on the causal connection are the same regardless of the meaning given to causation.
At present, it is probably the case in common law Canada that the first and second meanings do not produce different consequences. That is, the extent of liability as between plaintiff and defendant held liable will be the same, even where there is also contributory fault (contributory negligence) attributable to a plaintiff and, in those cases where there are multiple tortfeasors, the extent of contribution liability between the tortfeasors will be the same. In order for that to occur, apportionment doctrines would have to produce the same result under both meanings of causation. In principle, there is no reason for the result to be different. I am not aware of any case where the finding of causation had the second meaning – the legal fiction meaning – and anything about the result as between plaintiff and defendant, or between defendants, would have been different if the court had made a first meaning finding of factual causation- that the defendant’s tortious conduct was a cause of the injury on the balance of probability.
The key reason for that is that, in common law Canada, apportionment – whether for contributory fault purposes or contribution purposes – is based on the relative blameworthiness between the persons whose conduct has been held to be a cause of the losses for which damages were awarded. It is not based on some attempt to determine what part of the losses were caused by each of the persons whose conduct has been held to be a cause.
So let’s move to the third meaning.
Will that make a difference as between plaintiff and defendant, or as between defendants?
If it’s a policy-based reason for using the conclusion as part of the basis for the imposition of liability, then there should be policy-based reasons that could preclude the use of the conclusion for that purpose.
We had, in common law Canada, before Resurfice, only one instance that could produce the third meaning: a policy-based decision that the defendant’s tortious conduct will be treated as a cause of the plaintiff’s injury even though we admit that it cannot be validly shown that the tortious conduct was more likely than not a cause. That was the facts to which the rule in Cook v. Lewis  SCR 830, 1951 CAnLII 26 applied. But, Cook was sent back for a new trial and the Supreme Court did not deal with how contributory fault or contribution law would apply in the event the plaintiff succeeded, this time.
However, the facts of Cook v. Lewis reoccurred in 1958 in Ontario. The shot of one of two hunters injured a companion. The facts did not allow a valid conclusion as to which of the two fired the shot. Both were negligent. The injured person sued both. The result is reported: Lange v. Bennett  1 O.R. 233, 1963 CarswellOnt 137 (Ont. H.C.). The trial judge refused to apply Cook v. Lewis because the plaintiff was also at fault.
This is the headnote:
While on a hunting expedition with defendant and another, plaintiff knelt down to shoot at a bird. Defendant and the third person were behind plaintiff and shooting, from a standing position, at the same bird. Plaintiff suddenly stood up in the line of fire of his companions and was struck by a bullet fired by one of them. It was impossible in the circumstances to say which of the two fired the shot which struck plaintiff. In an action by plaintiff against one of his companions, held, the action should be dismissed. Where two or more persons acted negligently, but it was impossible to establish whose negligence caused the injury, both could be held liable; but that rule should not be extended to a case in which plaintiff had also been negligent. In such a situation, plaintiff must fail if he could not establish that it was defendant who caused his injury. In any event, plaintiff’s act of standing up in the direct line of fire of his companions was the effective, efficient cause of the accident.
Since all law students eventually realize not to rely on headnotes, this is the relevant part of the reasons – taken from the version of the reasons reported at 1963 CarswellOnt 137:
 I do not think that the legal principles that form the basis of the decision in Cook v. Lewis have any application to the facts before me. Negligence must necessarily be relative to the circumstances in each case and to the ages of the parties in question. The plaintiff at the time of the accident was two years older than the defendant. He kneeled down to shoot and knew that he was in the approximate line of fire of two younger boys, shooting from behind him. He was unquestionably negligent in standing up without warning and putting himself into the line of fire. He must be taken to have known that he might have been struck by a shot fired by either of them and it would be difficult to say in such circumstances which boy fired the shot that struck him. He had himself participated as a “negligent actor” in “the proof destroying fact”. To hold on these facts that the one who may have done him no harm should pay damages because he could not clear himself from blame would be grossly unjust. These are not special circumstances which render inapplicable the rule as laid down in Moxley v. Canada Atlantic Railway, supra, quoted with approval by Cartwright, J., in Cook v. Lewis. I have found no case where a plaintiff who was himself negligent has been able to avail himself of the doctrine of Summers v. Tice and I do not think the principles of Cook v. Lewis should be extended to such a case.
 In my opinion the action must therefore fail on the ground that the plaintiff has not established that the defendant fired the shot that injured him.
 Apart from what I have said, I think the action fails on the ground that it was the plaintiff’s own negligence that was the proximate, effective cause of the accident. As I have pointed out, he was two years older than the defendant. There is a wide difference in the mentality and judgment between boys of 16 years of age and boys of 18. The plaintiff, with full knowledge of the danger, negligently placed himself in the line of fire. He must be taken to have realized that the boys shooting from behind him would aim their rifles with one eye closed and the other on the sights, thus restricting their vision to a very narrow scope. Even if the younger boys were negligent in firing from behind the plaintiff, no injury would have come to him but for his own rash and thoughtless act. In these circumstances the questions propounded by Anglin, C.J.C., in Long v. McLaughlin, (sub nom. McLaughlin v. Long)  S.C.R. 303,  2 D.L.R. 186, “What was the proximate or efficient cause of such damage or loss?”, and Davis, J.A., in Falsetto v. Brown,  O.R. 645,  3 D.L.R. 545 (C.A.) (referred to by Hogg, J.A., in Partridge v. Etobicoke (Township) (1955),  O.R. 121, 1 D.L.R. (2d) 640 (C.A.), “Who was responsible for the accident?”, can only be answered by finding that the plaintiff’s own negligence in putting himself in the line of fire was the effective efficient cause of the accident.
I am aware of only one other attempt to use Cook v. Lewis in a case where the conduct of the plaintiff might have been considered relevant to the application of the doctrine. W was a “peeping tom”. He was spotted by two police officers. In the course of his attempt to escape, he was shot in the leg by one of the two officers. Both had fired their guns. The shot was not the result of a struggle between W and either of the officers. It was not possible to determine which of the two fired the shot. W sued both police officers. He succeeded. The trial judge held the officers were negligent in shooting, applied Cook v. Lewis, and held both defendants liable: Woodward v. Begbie,  O.R. 60, 31 D.L.R. (2d) 22, 1961 CarswellOnt 186 (Ont. H.C.).
This part of the reasons explains how W was shot and the trial judge’s conclusions in Woodward.
 … The defendants Yearwood and Begbie said that as they approached the plaintiff he started to run in a southwesterly direction toward the escape route open to him between the houses, no. 24 and 32 Rosewood. The defendants shouted to the plaintiff that they were Police Officers and called upon him to halt but he continued to run.
 Begbie said he fired a warning shot into the air and Yearwood said that almost simultaneously he fired a shot into the ground about 10 ft. to the left of the plaintiff for the purpose of turning the plaintiff toward the west or north and thus into the hands of the Police Officers approaching from Victoria Ave. The plaintiff continued on his course and the three officers pursued him and in a matter of seconds after the first shots were fired Begbie said he fired a second time into the ground and also about 10 ft. to the left of the plaintiff for the same purpose as Yearwood. Shortly thereafter the plaintiff changed his direction and was apprehended by the other officers. …
 I am also satisfied on the evidence that either Yearwood’s shot or Begbie’s second shot struck the plaintiff. The plaintiff said he thought it was one of the two simultaneous shots which struck him but his evidence on other matters is so incredible that I cannot place any reliance on this particular piece of evidence. There was no evidence of any change in pace in the plaintiff’s flight which might indicate which shot struck him and I am therefore unable to find whether it was Yearwood’s shot or Begbie’s second shot which caused the wound.
 As I have already said, it was either Yearwood’s shot or Begbie’s second shot which wounded the plaintiff and the plaintiff is entitled to succeed unless the defendants can establish that they did not intend to injure the plaintiff and that they were not negligent: Lewis v. Cook.
 As there is no evidence to show whether it was Yearwood’s or Begbie’s second shot that caused the wound, both are liable: Cook v. Lewis, supra.
Woodward does not contain any discussion of whether there was anything about the plaintiff’s conduct – the fact of his criminal act: either or both of his peeping and his attempt to escape arrest – that, as a matter of policy, could or should prevent the plaintiff from relying on the rule in Cook v. Lewis.
The trial judge in Lange v. Bennett did not refer to Woodward.
I’m not aware of another reported instance of the attempt to use Cook v. Lewis in a case where it could, in some sense, be said that the injured person was also at fault for the injury. If there is one, it’s not mentioned in Canada’s tort treatises (based on my recollection and my check of the material I have at home) AND is not mentioned in the online CED(3d) which cites Lange v. Bennett but no contrary cases.
A case that some might consider important, even though Cook v. Lewis was not applied is Hollis v. Dow Corning Corp  4 SCR 634, 1995 CanLII 55. The relevance of Cook, to this discussion, is captured in a passage from Klar, Tort Law (4th).
Due to the unusual features of the case, and the requirements of the rule, it is not surprising that the application of Cook v. Lewis has been infrequent. Although it has been applied in one case,in another case similar to Cook v. Lewis, the court refused to apply the rule on account of the plaintiff’s own negligence. In Hollis v. [Dow Corning Corp], the principle of Cook v. Lewis was reaffirmed, although the facts of the case bore no resemblance to those in Cook v. Lewis. The important point for the majority of the Court, however, was the principle that when a plaintiff is seriously disadvantaged in proving her case, through no fault of her own, the penalty should fall on the negligent defendant who created the problem.
[Internal footnotes omitted, emphasis added].
Now let’s go to the the explanation in Clements v. Clements, 2012 SCC 32 of the nature of the material contribution causation finding and the circumstances to which the material contribution doctrine applies.
 “But for” causation and liability on the basis of material contribution to risk are two different beasts. “But for” causation is a factual inquiry into what likely happened. The material contribution to risk test removes the requirement of “but for” causation and substitutes proof of material contribution to risk. …
. . . “material contribution” does not signify a test of causation at all; rather it is a policy-driven rule of law designed to permit plaintiffs to recover in such cases despite their failure to prove causation. In such cases, plaintiffs are permitted to “jump the evidentiary gap”: …
[46 (2)] Exceptionally, a plaintiff may succeed by showing that the defendant’s conduct materially contributed to risk of the plaintiff’s injury, where (a) the plaintiff has established that her loss would not have occurred “but for” the negligence of two or more tortfeasors, each possibly in fact responsible for the loss; and (b) the plaintiff, through no fault of her own, is unable to show that any one of the possible tortfeasors in fact was the necessary or “but for” cause of her injury, because each can point to one another as the possible “but for” cause of the injury, defeating a finding of causation on a balance of probabilities against anyone.
Let’s assume, too, that the Supreme Court did not mean, by the highlighted words in para.14, that all we are doing is substituting B for A, because, for law’s purposes, B means what A means.
One good reason for making that assumption is a statement in the next paragraph in the reasons.
 … As will become clearer when we discuss the cases, “material contribution” as a substitute for the usual requirement of “but for” causation only applies where it is impossible to say that a particular defendant’s negligent act in fact caused the injury. It imposes liability not because the evidence establishes that the defendant’s act caused the injury, but because the act contributed to the risk that injury would occur. …
Is that the better assumption of what the SCC meant? I think so.
Let’s now turn to real life (ignoring, for present purposes, the canard that nothing lawyers ever do is “real”).
P is injured in 3 separate incidents, involving three different negligent wrongdoers W1, W2, and W3 months apart. (I’ve chosen months for convenience. They could have been seconds, days, or years, just so long as the court is prepared to find that they are separate incidents.)
P is also at fault in at least one of the 3 incidents.
The action is tried at some point after the 3rd incident. The court finds that the significant portion of the compensable losses resulting from the 3 incidents are indivisible. That is, the court finds the conduct of each of P, W1, W2, and W3 could, on its own, be a but-for cause of the indivisible injuries portion of P’s injuries. Recall we’ve stipulated that this is the significant portion. If you want a real item to imagine, imagine this is the most of the past loss of income and all of the future loss of income, and a significant portion of the medical expense past and future. (Now you see why I put the incidents close together in time.)
Will a judge listen to a defence lawyer who says to the trial judge:
1. these facts fall under the Clements explanation of material contribution,
2. the plaintiff has been found to have also been at fault for the these losses,
3. therefore based on the Supreme Court’s explanation of the doctrine the plaintiff can’t succeed for these losses,
4. the action should be dismissed in respect of those losses?
Plaintiff’s lawyer will say: but-but-but- “indivisible injury” , the defendants are jointly liable, and cite the appropriate trial or appeal case for his or her province or territory. (Except in British Columbia, and maybe other provinces or territories other than Quebec, should the judges there decide the apportionment legislation means what the BC legislation means – proportional (several) liability where the injured person is also at fault).
Defendant’s lawyer will say: we have to presume the Supreme Court was aware of that law when it explained the meaning of material contribution. We have to presume that the Supreme Court did not intend to create contradictions and inconsistencies in the law. We have to take Clements as the law. Older law has to be understood in terms of Clements, not distinguished because trial or appellate judges, or plaintiffs, don’t like the consequences.
If the trial judge accepts the defence argument, what then happens when the judgment for the balance, however much it is, is less than the offers of settlement one or more of the defendants had made.
In any case, you know where we are ultimately bound don’t you? The banks of the Ottawa River, yet again. (More work for lawyers and judges; more ink to spill for commentators; who is to complain? Well … )
Principle might matter.
In any event, if we assume that the 3 different meanings have no different consequences, in law, after the decision that causation has been established, then the difference matters only to those who care about doctrinal coherence. Once upon a time, in a court that was open to admitting law’s warts, a judge of that court wrote:
No one can pretend that the existing law, which your Lordships have to accept, is founded upon principle. .. Consequently your Lordships are now engaged, not in the bold development of principle, but in a practical attempt, under adverse conditions, to preserve the general perception of the law as system of rules which is fair between one citizen and another.
White v. Chief Constable of South Yorkshire Police,  UKHL 45 at para. 48,  2 A.C. 455 at 511.