Black Holes, Aether – Excerpt 3 NESS and Sufficiency: A Better Test Than But-For

This is Part 9.c of “Black Holes, Aether“. As usual, the footnotes have been renumbered and modified to make the excerpt sufficiently self-contained.

There is a test for factual causation which better, broader, and far easier to use than the but-for test. It is better and broader because the test is capable of identifying all known instances of factual causation. It is easier to use because it does not require the judge or jury to undertake the metaphysical “what would have happened if” counterfactual analysis which is the defining characteristic of the but-for test. The test is known by the acronym “NESS”. The acronym stands for Necessary Element of a Sufficient Set.[1] As the acronym indicates, the NESS test is based on the premise that there is a set of factors which cumulatively make up a cause of the injury and the defendant’s negligence is a necessary element of that set. One seminal distinction between the NESS test and the but-for test is that the “is the negligence of this defendant a cause the plaintiff’s injury” question is answered by asking if the set of factors that necessarily includes the negligence is sufficient to have been a cause of the injury. If the answer to that question is yes, then the negligence of the defendant is a factual cause. The consequence is that the NESS test allows the existence of more than one sufficient causal set. It contemplates the existence of multiple sufficient causes. The “pointing finger” problem that troubled the Supreme Court in Clements does not arise except in the limited situation of cases where the causal alternatives are true alternatives – alternatives in the sense that if any one was a factual cause then all of the others were, in fact, not.

In the vast majority of these cases, that problem of alternative – or superseding – causes will be obvious because the occurrence of one of the causal candidates will have pre-empted – think prevented – the occurrence of any of the other causal candidates. For example, imagine a truck driving westerly along the main Ontario provincial highway between Toronto and Detroit. There are a number of bridges along that route. In some cases, there are no exits between the bridges. If one is stuck between the bridges, one is stuck until traffic clears ahead, or, I necessary, behind. Imagine that the truck Is due in Detroit at a particular time. It is carrying time sensitive material. Once loaded and in transit, there is no practical way to get that cargo to Detroit except on the truck. The truck is caught in a traffic jam east of Bridge 1. The underpass at Bridge 1 is blocked because of the fault of W1. There is no way off the highway for westbound traffic except at the Bridge 1 exits. The time required to clear the problem is long enough that, by the time the truck gets past Bridge 1, it is too late for the truck to get the cargo to Detroit on time. However, even if the problem at Bridge 1 had been fixed soon enough, the truck would have been blocked at the next bridge – Bridge 2 – by another problem. Assume that problem is the fault of W2. That problem would have prevented the truck from reaching Detroit on time even if the Bridge 1 problem had never occurred.

Can W1 and W2 point there fingers at one another? Is this an instance to which the material contribution test as explained in Clements applies? Can each of W1 and W2 say that their negligence is not a but-for cause? The answer should be that the negligence of W1 pre-empted the negligence of W2. In causation jargon, the negligence of W2 never instantiated as a cause. This answer is clear under the NESS test because the truck never reached Bridge 2 in time. The NESS test is based on what actually happened. There was never a causal set of facts that includes the truck arriving at Bridge 2 at a time when, had the underpass been clear, the truck could have passed in time to make Detroit as scheduled. The problem caused by the fault of W2 never came into play – never instantiated. The answer should be the same under the but-for test. That is, the court should find the problem caused by the fault of W2 was never a cause of the delay because it was pre-empted by the earlier problem caused by the fault of W1. The difference between this example, and Cook v. Lewis, is that we know who fired the shot that actually caused the problem that led to the injury. We can alter the facts of Cook v. Lewis to make them an instance of the truck example. If we assume that the two hunters had fired at sufficiently one after the other, and there is a witness (who is believed) who testifies that he or she heard both shots, and saw the plaintiff fall down before the second shot, then we have circumstantial evidence pointing to the right shooter. It does not matter that the ammunition from the guns is identical. It does matter that nobody saw the bullet hit the plaintiff.

What about multiple tortfeasor, duplicative negligent omission cases? In the duplicative omission case, “duplicative” means the multiple omissions are not cumulatively necessary. Rather, each of the omissions, by itself, is capable of being a but-for cause even if all of the other omissions had not occurred.

How would the Supreme Court suggest Canadian courts are to handle are to handle the facts of cases such as Saunders System Birmingham Co. v Adams.[2] In Saunders System, a garage (G) was negligent in repairing the brakes of B’s car. B was subsequently involved in a collision with C’s vehicle. B was negligent. He was not paying attention. He failed to apply the vehicle brakes at all. However, even had he applied the brakes, timely, that act would not have made a difference. The brakes had failed, earlier, due to the negligence of the garage. B did not know that, nor was there any reason for him to have known that. The brakes had worked properly when he had last applied them. In light of Clements, what would the test be for causation if those facts occurred in any of Canada’s common law jurisdictions and C sued B and G? Will B be able to argue, successfully, that the but-for test should apply? Will B be permitted to argue that his negligence was not necessary – did not make a difference – because even if he had not been negligent, if he had been paying attention and applied the brakes, the accident would still have happened because his brakes had failed? That is, will B be permitted to argue, paraphrasing Athey, that his negligence was not a “necessary contributing cause” since it was not necessary to have both his negligence and the negligence of the garage, given that even if he had applied the brakes properly the accident would still have occurred? [3] If B is, then B escapes liability.

The garage, of course, cannot point its finger at the driver, but this is because of the rules governing legal responsibility, not factual causation. B was entitled to assume that G had properly repaired the brakes. It was, or ought to have been, foreseeable to G that the brakes might fail. It would not be a defence for G that B had not attempted to use the brakes. As such the facts are not similar to Walker Estate where the CRCS could point its finger – in an attempt to break the chain of legal causation – at the donor.

The driver would, of course, escape liability if he had not been negligent where the driver had attempted, properly, to apply the brakes. However, the orthodox but-for test seems to require that the driver escape liability because his negligence was not necessary, whether he was negligent or not. B, of course, escapes liability if he was not negligent, but that reason does not provide an answer for the situation where B is negligent. Another way to describe this result is that the collision was inevitable, notwithstanding B’s negligence.[4] There is implicit Supreme Court of Canada authority for that result. In Rintoul v. X-Ray and Radium Industries Ltd.[5] In Rintoul, the brakes were working, seemingly properly, immediately before the accident. However, the brakes failed when the driver applied them indenting to stop his vehicle before it struck a stationary car ahead of him. The driver and owner pleaded “inevitable accident”. The defence succeed at trial and on appeal. The Supreme Court of Canada allowed the plaintiff’s appeal. The reason was the owner had not adduced evidence as that the brakes had been properly maintained nor why the brakes failed – the evidence required for the conclusion that the failure of the brakes was inevitable[6] – and the driver had not adduced evidence that, even after the brakes failed, he “could not by the exercise of reasonable care have avoided the collision.”[7]

The seeming difference between Rimtoul and the example based on Saunders System is that the driver in Rimtoul applied his brakes, albeit the brakes did not work. Should that be a relevant distinction? In Rimtoul, the driver was also negligent in what he did after he applied the brakes and found that they were not working properly. “Accepting the evidence of Ouellette as to the speed and position of his car at the instant he actually applied the hand brakes, it is obvious that if they had been in the state of efficiency prescribed by the regulations he could have stopped his car before the collision occurred, even if the car had not been, as it was, proceeding uphill.”[8] That statement necessarily means the Court did not believe the driver’s evidence that he had used the hand brakes. Recall that there was no evidence about brake maintenance; therefore, the Court must have presumed that the hand brakes were working properly. But, the conclusion that the proper use of a working hand brake would have stopped the vehicle in time means the driver’s negligence (B in example I have used) was an independent cause of the collision. It would have still been sufficient to cause the collision even if the main brakes had not failed but were not used because the driver did not apply them. So, in orthodox but-for terms, the collision in Rintoul, as it actually occurred, had two sets of events each capable of being a but-for cause: (1) the owner’s failure to maintain the vehicle’s main brakes, on basis that the driver’s use of the brakes would have stopped the vehicle had the brakes been working; and (2) the driver’s failure to apply the hand brake, on basis that the driver’s use of the hand brakes would have stopped the vehicle had the hand brakes been working.

But, putting aside the responsibility issue which is owner’s vicarious liability for the negligence of the driver – because it is irrelevant to the factual causation issue – the question relating to the but-for test applied to the owner’s failure to maintain the main brakes is: if the driver’s later, proper, application of the hand brake, had that occurred, would have prevented the collision from occurring at all, then how can it be said that the earlier negligence of the owner in failing to maintain the brakes was necessary for the collusion to have occurred? Logically, it cannot. The policy-based answer is that the garage in Saunders System and the owner in Rintoul are not permitted to make that argument because, as a matter of legal responsibility, it does not matter that the driver’s subsequent negligence supervened; that the driver’s negligence, considered separately, meant that the prior negligence was not a necessary cause. Or, putting this another way, it does not necessarily matter to the responsibility question, as applied to the negligence of the earlier actor, that there is later conduct which supervenes, so long as that later conduct is negligent, if it is the conduct of another person other than the injured person, or it is the contributory fault of the injured person.[9] The rules of the responsibility issue may be such as to prevent any of the negligent persons from arguing that somebody else was also at fault as a defence to the injured person’s claim, regardless of whether that other negligence could be a but-for cause, if there were no other negligence. This also seems to be the answer required by the principles that determine the rules that apply in the successive accident cases where each of the multiple acts of negligence is separately sufficient to have caused at least some of the same injury alleged by the plaintiff, or different injury but some overlapping (same) loss.[10]

The rule that a defendant cannot argue that there is somebody else also at fault, whether that person’s negligence is a separately sufficient cause, or cumulatively necessary with the negligence of the defendant does not flow from the rule that negligent person can never have a defence based on the proposition that non-negligent conduct of some other person was also necessary for the injury.[11] If that were a defence, there could never be any liability because there are always non-negligent factors which are necessary constituents of the set of events, of which the defendant’s negligence is a part, that is a cause of the injury. Athey made this point clearly. It is sufficient that the defendant’s negligence be “part of the cause …. even though his act alone was not enough to create the injury.”[12] It is sufficient that the negligence is necessary for the occurrence of the injury even if the negligence “played a minor role”.[13]

These complications in the use of the but-for test do not occur under NESS. Where the driver does not attempt to apply the vehicle’s brakes, or attempts to use them in a way that that makes the lack of maintenance irrelevant, the correct answer under NESS is that the driver cannot point to the garage’s, or owner’s , because the brake problem was never a part of the set of facts that included the driver’s negligence as a necessary component of the facts explaining how the accident actually occurred. From the driver’s perspective, the brake problem did not play any part in the events, as the events occurred, because the driver never applied the brakes, or never attempted to apply them in a way that the brakes would have stopped the vehicle had the brakes been functioning properly. Therefore, the fact that the brakes would not have worked, had the driver applied them, is irrelevant.[14] In the terminology of causation literature, the brake problem did not “instantiate”.[15]

The NESS test becomes the equivalent of the but-for test where the only question the judge or jury has to decide is whether to the evidence is enough to permit a valid finding, on the balance of probability, that the negligence of the defendant did cause the plaintiff’s injury and where there is no alternative proffered for the defendant’s negligence. In those cases, because there is no proffered alternative, the judge or jury is not asked to perform the counterfactual analysis which is the hallmark of the but-for test. The judge or jury need only decide whether the evidence is sufficient for the decisions that the defendant’s negligence was a necessary part of the causal set.[16]

There is irony in the Supreme Court’s seeming insistence, without explanation,[17] that only valid method of establishing the existence of factual causation, on the balance of probability, is the orthodox but-for test, while concurrently adopting what amounts to the NESS test as the basis of for determining if instances of two or more tortfeasors create qualifying impossibility. Recall how Clements describes the problem in the case that has qualifying impossibility.

         Typically, there are a number of tortfeasors. All are at fault, and one or more has in fact caused the plaintiff’s injury. The plaintiff would not have been injured “but for” their negligence, viewed globally. …

The plaintiff effectively has established that the “but for” test, viewed globally, has been met. It is only when it is applied separately to each defendant that the “but for” test breaks down because it cannot be shown which of several negligent defendants actually launched the event that led to the injury.[18]

Applying the but-for test globally amounts to not applying the but-for test separately. Not applying the but-for test separately in the specific causation inquiry – in the inquiry as to whether the negligence of this defendant was a cause of the injury – amounts to not applying the counterfactual portion of the but-for test which is the method used to determine if the alleged negligence was necessary for the occurrence of the injury. Not determining whether the negligence was necessary for the occurrence of the injury amounts to determining whether that negligence was sufficient, with other facts, to be a factual cause of the injury on the balance of probability. That, of course, is the NESS test.

The irony, of course, is that if the NESS test validly shows the existence of factual causation on the balance of probability – and it does – then resort to material contribution is not necessary in all cases to which the NESS test validly applies. More than that, resort to the material contribution is wrong because it should be understood to be premised on the concept that the evidence does not permit the valid identification of any negligence as a factual cause on the balance of probability. But that is not the case where the NESS test validly applies and identifies duplicative factual causation. Had the Supreme Court not been so caught up in the need to save the but-for test from the onslaught of academics, real and manqué, intent on spilling ink, perhaps the Court would have seen this. Perhaps the Court would have seen that the only problematic instance of uncertainty in factual causation actually caught by its explanation of qualifying impossibility is the situation having alternative (superseding) factual causes.


[1] Lewis Klar, Tort Law, 4th ed. (Toronto: Carswell, 2008) at 430, n. 11, provides a brief outline of the NESS test. The seminal article is Richard W. Wright, “Causation in Tort Law” (1985), 73 Calif. L. Rev. 1737. The updated statement is found in Richard W. Wright, “The NESS Account of Natural Causation: A Response to Criticisms”, in Perspectives on Causation, ed. R. Goldberg (Oxford: Hart Publishing, 2011), 285 at 286, n. 9 [Wright, “NESS Account”].

[2] 217 Ala. 621, 117 So. 72 (Ala. 1928) [Saunders System].

[3] See Athey, [1996] 3 S.C.R. 458 at  para. 41.2, 1996 CanLII 183 [Athey]

[4] Ibid., para. 41.2

[5] [1956] S.C.R. 674, 1956 CanLII 16 [Rintoul].

[6] Ibid. at 679.

[7] Ibid. at 681.

[8] Ibid. at 680.

[9] Whether it does matter will be a remoteness (scope of liability) issue, not factual causation issue. See Part 5.d [Scope of Liability … Remoteness] … for limited discussion of the remoteness issue.

[10] See Klar, Tort Law, supra note 1 at 460-61, 465 para. (vi).

[11] Athey, supra note 2 at para. 19. Hence Athey, para. 41.1, states that causation is not proven under the but-for test if the injury would have occurred as it did, implicitly due to some not-tortious cause, even if the negligence had not occurred. This blanket proposition is not applied where the other sufficient cause is the injured person’s contributory fault. Athey, para. 41.1 has to be understood as referring only to the but-for test given para. 41.3. The meaning of para. 41.3 is not clear. The better interpretation is that it refers to duplicative causation and the remoteness (scope of responsibility) issue. See Part 5.d (Remoteness) and supra at the text accompanying notes 504-509. It cannot mean cases where the evidence is not enough to allow any decision on the balance of probability. The alternative is that the passage refers to alternative causation in a situation where the evidence is not sufficient to allow a valid conclusion as to which of the choices is the probable cause. However, to conclude the section applies to all instances of alternative causation is to make it apply to Cook v. Lewis [1951] S.C.R. 830, 1951 CanLII 26 instances. That is problematic. If it applies to duplicative causation, though, it is not problematic because one of the sufficient causes might be more significant in a normative sense. Is there room for this in Clements?

[12] Ibid. at para. 17 [emphasis in original].

[13] Ibid. at para. 41.2.

[14] See Richard W. Wright, “Acts and Omissions as Positive and Negative Causes” in Emerging Issues in Tort Law, J. Neyers, E. Chamberlain, and S. Pitel, eds. (Oxford: Hart Publishing, 2007), at 302-05 [Wright, “Acts and Omissions”].

[15] Ibid. at 293-94. Wright states, ibid.:

“In causal analysis, the relevant set is the antecedent (‘if’ part) of a causal law. A causal law is a statement that describes an empirically derived relation between a set of conditions (called the antecedent) and a condition (called the consequent) such that the complete instantiation of all the conditions in the antecedent on a particular occasion is sufficient for the instantiation of the consequent, the instantiation of which occurs subsequent to (or perhaps simultaneously with) the instantiation of all the conditions in the antecedent. Thus, a condition was a cause of some consequence if and only if it was part of the complete instantiation of the antecedent of a causal law that specifies … ‘the sum total of the conditions positive and negative taken together; the whole of the contingencies of every description, which being realised, the consequent invariably follows’.”

[Internal footnotes omitted.] See also Wright, “Ness Account”, supra note 649 at 289 (at the text accompanying nn. 24-28) and 292 (text accompanying nn. 40-41). Wright’s current summary in “NESS Account”, ibid. at the text accompanying nn. 40-41, is:

“When analyzing singular instances of causation, an actual condition c was a cause of an actual condition e if and only if c was a part of (rather then being necessary for) the instantiation of one of the abstract conditions in the completely instantiated antecedent of a causal law, the consequent of which was instantiated by e immediately after the complete instantiation of its antecedent, or (as is more often the case) if c is connected to e through a sequence of such instantiations of causal laws. … [C]are must be taken to make sure that the antecedents of the relevant causal laws include only abstract NESS conditions in the strict sense—that is, those abstract conditions the instantiation of which is necessary for the sufficiency of the set of conditions that is sufficient for the instantiation of the consequent.”

[Underlining emphasis added; internal footnotes omitted.]

[16] See Wright, “Acts and Omissions”, supra note 14 at 295, at the text accompanying n.23: “Rather than being the exclusive test of causation, as some have argued or assumed, the necessary-condition test works only as an inclusive test of causation, in situations in which (as is usually true) there was only one set of antecedent conditions that was or would have been sufficient for the occurrence of the consequence on the particular occasion, or, if there was more than one such set, the condition was necessary for the sufficiency of each of the sets. In such situations, the NESS test reduces down to the necessary-condition test.” See also Wright, “NESS Account”, supra note 649 at 292 (at the text accompanying nn. 42-45). “The sine qua non … analysis, properly applied, is a corollary of the NESS analysis that gives the correct answer when there was only one set of conditions that was actually or potentially sufficient for the consequence on the particular occasion.” [Internal footnotes omitted.]

[17] One explanation might be a belief that restricting the number of available tests for proof of factual causation will simplify and clarify doctrine, so make life that much easier for lawyers and judges who have to decide what the applicable law is.

[18] Clements v. Clements, 2012 SCC 32  at paras. 39-40. See also para. 39.

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