Causation. Sack v Ross
In Sack v Ross (Ont CA, 2017) the Court of Appeal engages in an extensive consideration of principles applicable to causation in negligence, and the sometimes difficulty of applying the standard 'but for' causation test when dealing with: an omission rather than an action, several alleged tortfeasors, and delayed diagnosis and treatment:
E. Issue One: Did the trial proceed on a correct understanding of causation in negligence cases?. Martin-Vandenhende v. Myslik
 The common law feels its way on a case by case basis recognizing that judges are not omniscient. A carefully formulated legal proposition that works justice in a specific case might later be found, in the light of different facts, to be over-inclusive or under-inclusive, or as otherwise flawed or inadequate. The paradoxical genius of the common law is that, on principle, the process of refinement and adaptation is interminable.
 The doctrine of causation in negligence is a signal example. As McLachlin C.J. observed in Resurfice Corp. v. Hanke, 2007 SCC 7 (CanLII), at para. 20: “Much judicial and academic ink has been spilled over the proper test for causation in cases of negligence.” The Supreme Court’s decision in Clements v. Clements, 2012 SCC 32 (CanLII),  2 S.C.R. 181 ushered in another wave of ink.
 In picking through the law relating to causation in negligence actions, there is some conceptual clarity to be gained by distinguishing between simple negligence cases involving a plaintiff and a single defendant, and complex cases involving multiple tortfeasors.
(1) Simple Negligence Actions
 The paradigm negligence case is one in which a single defendant is alleged to have negligently done something that caused the plaintiff’s injury. To succeed the plaintiff must prove both the defendant’s breach of the standard of care and causation.
 The current state of the law regarding causation in negligence cases was set out by McLachlin C.J. in Clements at para 8:
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 by McLachlin C.J.] The determination of causation is understood as a factual inquiry. In all instances of determining causation, Professor Russell Brown (as he then was) observed that the trier of fact proceeds by inference and not as a direct witness of the events: “The Possibility of “Inference Causation”: Inferring Cause-in-Fact and The Nature of Legal Fact-Finding” (2010) 55 McGill L.J. 1; “Known Unknowns in Cause-in-Fact” (2010-2011), 39. Adv. Q. 37; “Cause-in-Fact at the Supreme Court of Canada: Developments in Tort Law in 2012-2013”, (2014), 64 S.C.L.R. (2d) 327.
 Very few fact situations demonstrate the causal clarity of a game of billiards in which the combinations of balls striking balls can be easily replicated. The closest equivalent would be a simple case involving a single defendant who did something in breach of the standard of care that physically hurt the defendant, where the “but for” test is relatively easy to apply. Consider the cases in which the defendant accidently shoots the plaintiff in a hunting mishap, or where the defendant runs a red light and collides with the plaintiff who suffers physical trauma. Causation is not usually a live issue in such cases because the causal inferences are so easy to draw.
 Things are more complicated where the complaint is not about something the defendant did, but about something the defendant failed to do in breach of the standard of care. When what is in issue is not the defendant’s act, but an omission, the trier of fact is required to attend to the fact situation as it existed in reality the moment before the defendant’s breach of the standard of care, and then to imagine that the defendant took the action the standard of care obliged her to take, in order to determine whether her doing so would have prevented or reduced the injury. Even though this exercise is bounded significantly by the actual facts, it counts as “factual” because the task is to consider how the events would actually have unfolded had the defendant taken the action she was obliged to take.
(2) The Causal Reasoning Process
 Regardless of whether the defendant’s breach of the standard of care is an act or an omission, the trier of fact’s cognitive process in determining causation has three basic steps. The first is to determine what likely happened in actuality. The second is to consider what would likely have happened had the defendant not breached the standard of care. The third step is to allocate fault among the negligent defendants.
 There are two possible outcomes to the trier of fact’s imaginative reconstruction of reality at the second step. On the one hand, if the trier of fact draws the inference from the evidence that the plaintiff would likely have been injured in any event, regardless of what the defendant did or failed to do in breach of the standard of care, then the defendant did not cause the injury. On the other hand, if the trier of fact infers from the evidence that the plaintiff would not likely have been injured without the defendant’s act or failure to act, then the “but for” test for causation is satisfied: but for the defendant’s act or omission, the plaintiff would not have been injured. The defendant’s fault, which justifies liability, has been established.
 I move now to consider some more factually complicated situations.
(3) Negligence Actions Involving Multiple Defendants
 Many factual combinations and permutations are possible in negligence cases, including the complexities arising from multiple negligent parties, multiple potential causes, successive and cumulative injuries, divisible and indivisible injuries, injury aggravation, and the thin and crumbling skull scenarios: see Erik Knutsen, “Clarifying Causation in Tort”, (2010) 33 Dal. L.J. 153, and “Coping with Complex Causation Information in Personal Injury Cases”, (2013) 41 Adv. Q. 149.
 Delayed diagnosis medical negligence cases involving multiple tortfeasors typically raise several such complexities, and are among the most complex to assess from the perspective of causation.
 Chief Justice McLachlin took the view in Clements that the basic “but for” test also applies in these more complex cases, noting at paras. 12 and 43:
In some cases, an injury -- the loss for which the plaintiff claims compensation -- may flow from a number of different negligent acts committed by different actors, each of which is a necessary or "but for" cause of the injury. In such cases, the defendants are said to be jointly and severally liable. The judge or jury then apportions liability according to the degree of fault of each defendant pursuant to contributory negligence legislation. In his 2010 article, at p. 170, Professor Knutsen made the following observations, basically agreeing with McLachlin C.J.:
It is important to reaffirm that in the usual case of multiple agents or actors, the traditional "but for" test still applies. The question, as discussed earlier, is whether the plaintiff has shown that one or more of the defendants' negligence was a necessary cause of the injury. Degrees of fault are reflected in calculations made under contributory negligence legislation.
“But for” causation is therefore not complicated in successive or cumulative injury context when one focusses solely on each actor and each actor’s role in a causal story. The confusion results when one forgets two things: first, that it is a defendant’s breach of the standard of care that is the locus of the causal inquiry, and second, that a defendant’s negligence need only be proven to be “a” cause of “some” injury to the plaintiff. However, it is worth observing that the Supreme Court has never considered cases beyond the simple. Even in Athey v. Leonati, 1996 CanLII 183 (SCC),  3 S.C.R. 458, which involved successive accidents with different defendants, the plaintiff’s injuries were treated as a single tort by agreement of the parties, which greatly simplified the analysis. I do not consider Cook v. Lewis 1951 CanLII 26 (SCC),  S.C.R. 830, which was a very strange case about a hunting accident, to be an example applicable to conventional delay of diagnosis and treatment cases.
 As I explain below, this case shows that the trier of fact’s cognitive process in reasoning through causation in a delayed diagnosis and treatment case does not unfold as simply as these statements of the doctrine might suggest.
In Martin-Vandenhende v. Myslik (Ont CA, 2015) the Court of Appeal briefly set out the correct approach to considering causation in negligence:
 The basic test for determining causation is the “but for” test: Clements v. (Litigation Guardian of) Clements, 2012 SCC 32 (CanLII),  2 S.C.R. 181, at para. 8. The plaintiff must show that “but for” the defendant’s negligent act, the injury would not have occurred. Only if “special circumstances” make it impossible to prove “but for” causation and if applying the test “would offend basic notions of fairness and justice” will the “material contribution” test apply: Hanke v. Resurfice Corp., 2007 SCC 7 (CanLII),  1 S.C.R. 333, at paras. 24-25; Clements, at paras. 27-28.. Benhaim v. St‑Germain
In Benhaim v. St‑Germain (SCC, 2016) the Supreme Court of Canada canvassed basic principles of causation in the context of medical malpractice, including the situation where a party's inability to prove a fact is due to the negligence or other behaviour of another party ("spoliation"):
 As I will now explain, Snell and St-Jean held that the ordinary rules of causation must be applied in medical malpractice cases. As prime examples of how the ordinary rules of causation operate in medical liability cases, these decisions are equally relevant in Quebec.. Jarbeau v. McLean
 I will begin by discussing Snell, in which Sopinka J. examined developments in English tort law that purported to reverse the onus of proving causation in some circumstances. Traditionally, the plaintiff in a common law negligence claim had to prove on a balance of probabilities that, but for the defendant’s negligent conduct, the plaintiff would not have been injured. Sopinka J. stated that his task was “to determine whether a departure from well-established principles is necessary for the resolution of this appeal” (p. 320). Sopinka J. concluded that such a departure was not warranted, provided that the traditional principles are not applied in an overly rigid manner (p. 328).
 First, Sopinka J. held that it is not necessary that the plaintiff adduce expert scientific or medical evidence definitively supporting the plaintiff’s theory of causation, as “[c]ausation need not be determined by scientific precision” (p. 328; see also pp. 330-31). This is because the law requires proof of causation only on a balance of probabilities, whereas scientific or medical experts often require a higher degree of certainty before drawing conclusions on causation (p. 330). Simply put, scientific causation and factual causation for legal purposes are two different things. Factual causation for legal purposes is a matter for the trier of fact, not for the expert witnesses, to decide: Laferrière v. Lawson, 1991 CanLII 87 (SCC),  1 S.C.R. 541, at pp. 607-8; see also Sentilles v. Inter-Caribbean Shipping Corp., 361 U.S. 107 (1959), at pp. 109-10.
 Second, in medical malpractice cases, the defendant is often in a better position than the plaintiff to determine the cause of the injury (p. 322). Sopinka J. held that, in such circumstances, the trier of fact may take into account the relative ability of each party to present evidence on a fact in issue:
The legal or ultimate burden remains with the plaintiff, but in the absence of evidence to the contrary adduced by the defendant, an inference of causation may be drawn although positive or scientific proof of causation has not been adduced. If some evidence to the contrary is adduced by the defendant, the trial judge is entitled to take account of Lord Mansfield’s famous precept. [p. 330]This precept, stated by Lord Mansfield in Blatch v. Archer (1774), 1 Cowp. 63, 98 E.R. 969, at p. 970, is that evidence should be “weighed according to the proof which it was in the power of one side to have produced, and in the power of the other to have contradicted” (quoted in Snell, at p. 328).
 An inference of causation is available to trial judges by virtue of the ordinary operation of these principles in the medical malpractice context:
In many malpractice cases, the facts lie particularly within the knowledge of the defendant. In these circumstances, very little affirmative evidence on the part of the plaintiff will justify the drawing of an inference of causation in the absence of evidence to the contrary. The majority of the Court of Appeal relied in large part on this passage to justify its conclusion that Snell created a rule of law that requires an adverse inference of causation in certain circumstances. The inference of causation Sopinka J. described in Snell is one that trial judges are permitted to draw even in the absence of positive or scientific proof. It is not one that they are required to draw once certain criteria are established. The decision on whether to draw such an inference is left to the discretion of the trial judge. Despite using permissive language to describe the adverse inference in Snell, the decision of the majority of the Court of Appeal failed to give effect to the permissive, discretionary nature of that inference.
(Snell, at pp. 328-29)
 By overturning the trial judge’s decision on the basis of an error of law, the majority of the Court of Appeal implicitly transformed the permissive inference described by this Court in Snell into one that is compulsory once certain facts are established. The majority’s decision would have the effect of creating a novel legal rule governing presumptions. And yet it is apparent that Sopinka J. was not purporting to create such a rule in Snell. Rather, he was simply describing how the usual fact-finding process works in the medical malpractice context:
It is not strictly accurate to speak of the burden shifting to the defendant when what is meant is that evidence adduced by the plaintiff may result in an inference being drawn adverse to the defendant. Whether an inference is or is not drawn is a matter of weighing evidence. The defendant runs the risk of an adverse inference in the absence of evidence to the contrary. This is sometimes referred to as imposing on the defendant a provisional or tactical burden. . . . In my opinion, this is not a true burden of proof, and use of an additional label to describe what is an ordinary step in the fact-finding process is unwarranted. [Emphasis added; pp. 329-30.]. The adverse inference of causation described in Snell is permissive precisely because it is a component of the fact-finding process. For the same reason, the question of whether an inference is warranted in a particular case falls within the discretion of the trier of fact, to be determined with reference to all of the evidence. This principle was recently reaffirmed by this Court in Ediger, at para. 36. It was therefore not open to the majority to substitute its own decision to draw an unfavourable inference. Rather, the majority of the Court of Appeal would have been bound to find a palpable and overriding error in the trial judge’s decision not to draw an adverse inference. I return to this point below.
 Snell itself provides an example of the circumstances in which an inference of causation may be drawn. The plaintiff sued the defendant ophthalmologist after she became blind in her right eye following cataract surgery. Before the surgery, the defendant noticed some bleeding when he injected anaesthetic, yet he negligently continued the surgery. The blindness could have been caused by the bleeding or by natural causes. The expert witnesses could not definitively determine the cause. In concluding that the plaintiff had established that the defendant’s negligence caused her blindness, Sopinka J. stated:
The [defendant] was present during the operation and was in a better position to observe what occurred. Furthermore, he was able to interpret from a medical standpoint what he saw. In addition, by continuing the operation which has been found to constitute negligence, he made it impossible for the [plaintiff] or anyone else to detect the bleeding which is alleged to have caused the injury. In these circumstances, it was open to the trial judge to draw the inference that the injury was caused by the retrobulbar bleeding. There was no evidence to rebut this inference. . . . In sum, the Court held in Snell that “the plaintiff in medical malpractice cases — as in any other case — assumes the burden of proving causation on a balance of the probabilities”: Ediger, at para. 36. Causation need not be proven with scientific or medical certainty, however. Instead, courts should take a “robust and pragmatic” approach to the facts, and may draw inferences of causation on the basis of “common sense”: Snell, at p. 330-331; Clements, at paras. 10 and 38. The trier of fact may draw an inference of causation even without “positive or scientific proof”, if the defendant does not lead sufficient evidence to the contrary. If the defendant does adduce evidence to the contrary, then, in weighing that evidence, the trier of fact may take into account the relative ability of each party to produce evidence: Ediger, at para. 36.
. . . it is not essential to have a positive medical opinion to support a finding of causation. Furthermore, it is not speculation but the application of common sense to draw such an inference where, as here, the circumstances, other than a positive medical opinion, permit. [pp. 335-36].
In Jarbeau v. McLean (Ont CA, 2017) the Court of Appeal discusses the 'but for' test in the context of an extended discussion of solicitor's negligence:
 The “but for” test set out in the trial judge’s charge is the appropriate test for causation in negligence in all but rare cases. The test was set out in Clements v. Clements, 2012 SCC 32 (CanLII),  2 S.C.R. 181, at para. 8, as follows:
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.]