2. 'But for' and Complications
3. Significant Contributing Cause
4. Break in Chain of Causation (Novus Actus Interveniens)
1. GeneralThe accepted wisdom on 'causation' was, and mostly is, the 'but for' standard. It stems from the philosophy that delineates 'necessary' and 'sufficient' causes. A 'necessary' cause is one that amongst others has to be present for the effect to take place, and a 'sufficient' cause is one that by itself can create the effect. A 'but for' cause is a human-created necessary cause.
Mind you, with law being a thoroughly social activity, with social mores at it heart, it's not always quite so simple. Causation is inherently a social creation, not a feature of physics. So, as for example with the consideration of 'omissions', the identification of cause in law remains a sometimes complex matter.
2. 'But for' and Complications. Sack v Ross
In Sack v Ross (Ont CA, 2017), a complex medical malpractice case, which bears reading in full, 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 some situations. These situations are: 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?. White v. St. Joseph's Hospital (Hamilton)
 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 White v. St. Joseph's Hospital (Hamilton) (Ont CA, 2019) the Court of Appeal considers medical delay in the context of the but-for test for causation:
 In an action for delayed medical diagnosis and treatment, a plaintiff must establish that the delay caused or contributed to the unfavourable outcome: Sacks v. Ross, 2017 ONCA 773 (CanLII), 417 D.L.R. (4th) 387, leave to appeal to SCC refused, 2018 CarswellOnt 10678-10679, at para. 117; Beldycki Estate v. Jaipargas, 2012 ONCA 537 (CanLII), 295 O.A.C. 100, at para. 44. The phrase “caused or contributed” originates in the Negligence Act, R.S.O. 1990, c. N.1, at s. 1, and is the normative test applied by this court, as set out in Sacks v. Ross, at para. 117, and embodied in the “but for” test prescribed by the Supreme Court in Clements v. Clements, 2012 SCC 32 (CanLII),  2 S.C.R. 181, at para. 8. In other words, “but for” the alleged delay would the plaintiff have suffered the unfavourable outcome? (Nothing in Sacks v Ross revived the “material contribution to injury” test.) The trial judge found that Mr. White failed to prove that the delay in treatment caused or contributed to his injuries. She noted: “Dr. Fong was not able to articulate what injury he says was avoidable.” The trial judge stated: "Dr. Fong agreed that most of Mr. White's outcome was unavoidable. The court finds this evidence is devastating to the plaintiffs' case."
3. Significant Contributing Cause. R v Singh
In R v Singh (Ont CA, 2015), a criminal case against a landlord found to have caused the death of a tenant, the Court of Appeal endorsed the following definition of causation:
 As the trial judge correctly noted, the test for determining legal causation is whether the appellant’s conduct was a significant contributing cause of Mr. Dhaliwal’s death: R v. Nette, 2001 SCC 78 (CanLII),  3 S.C.R. 488, at paras. 69-72; R. v. Shilon, 2006 CanLII 41280 (ON CA), 240 C.C.C. (3d) 401,  O.J. No. 4896 (C.A.), at paras. 27, 30.
4. Break in Chain of Causation (Novus Actus Interveniens). Salomon c Matte-Thompson
In Salomon v. Matte‑Thompson (Ont CA, 2019) the Supreme Court of Canada commented as follows on when the chain of causation is broken:
 I also agree with the Court of Appeal that the fraud did not break the causal link between Mr. Salomon’s faults and the respondents’ losses. It is true that a person who commits a fault is not liable for the consequences of a new event that the person had nothing to do with and that has no relationship to the initial fault. This is sometimes referred to as the principle of novus actus interveniens: that new event may break the direct relationship required under art. 1607 C.C.Q. between the fault and the injury. Two conditions must be met for this principle to apply, however. First, the causal link between the fault and the injury must be completely broken. Second, there must be a causal link between that new event and the injury. Otherwise, the initial fault is one of the faults that caused the injury, in which case an issue of apportionment of liability may arise (Baudouin, Deslauriers, Moore, at Nos. 1-691 to 1-692; Laval (Ville de) (Service de protection des citoyens, département de police et centre d’appels d’urgence 911) v. Ducharme, 2012 QCCA 2122 (CanLII),  R.J.Q. 2090, at paras. 64-65; Lacombe v. André, 2003 CanLII 47946 (QC CA),  R.J.Q. 720, at paras. 58-60 (C.A.)).
Cases to be integrated