According to the most recent reminder from the Supreme Court of Canada, the application of common sense in a robust, pragmatic, manner, will provide the answer to the factual causation question – is the defendant’s negligence more likely than not a necessary cause of the injury – in most cases where a plaintiff alleges that the plaintiff’s injury was caused by negligence for which the defendant is responsible. See Clements v. Clements 2012 SCC 32 at paras. 9, 10, 23, 38, 46(1).
On the other hand, we must not forget that it was once “Western” common sense that witches existed; the Earth was flat, the centre of the universe, and the Sun rotated around the Earth; Heaven and Hell were physical places; diseases were caused by an imbalance of humours; if one could travel fast enough, one could catch light; women were the property of men; Homo Sapiens did not evolve from earlier forms of hominid but has always had its current form; and creation occurred on the night preceding Sunday, 23 October 4004 BCE (by the Julian calendar).
A good friend reminded me, recently, that there are parts of Canada where some the items in that list are taken as gospel. (He wasn’t referring to Ottawa.)
Clements v. Clements 2012 SCC 32:
 The “but for” causation test must be applied in a robust common sense fashion. There is no need for scientific evidence of the precise contribution the defendant’s negligence made to the injury. See Wilsher v. Essex Area Health Authority,  A.C. 1074, at p. 1090, per Lord Bridge; Snell v. Farrell, 1990 CanLII 70 (SCC),  2 S.C.R. 311.
 A common sense inference of “but for” causation from proof of negligence usually flows without difficulty. Evidence connecting the breach of duty to the injury suffered may permit the judge, depending on the circumstances, to infer that the defendant’s negligence probably caused the loss. See Snell and Athey v. Leonati, 1996 CanLII 183 (SCC),  3 S.C.R. 458. …
 This Court … [in Athey], discussed the limitations of the “but for” test and the propriety of exceptionally using a material contribution test. Major J. emphasized that a robust common sense approach to the “but for” test permits an inference of “but for” causation from evidence that the defendant’s conduct was a significant factor in the injury,
 … The law of negligence has never required scientific proof of causation; to repeat yet again, common sense inferences from the facts may suffice. …
(1) … As a general rule, a plaintiff cannot succeed unless she shows as a matter of fact that she would not have suffered the loss “but for” the negligent act or acts of the defendant. A trial judge is to take a robust and pragmatic approach to determining if a plaintiff has established that the defendant’s negligence caused her loss. Scientific proof of causation is not required.
K.L.B. v. British Columbia,  2 SCR 403 at para. 13, 2003 SCC 51:
 … As in other areas of negligence law, judges should assess causation using what Sopinka J., citing Lord Bridge in Wilsher v. Essex Area Health Authority,  2 W.L.R. 557 (H.L.), at p. 569, referred to as a “robust and pragmatic approach” (Snell v. Farrell, 1990 CanLII 70 (SCC),  2 S.C.R. 311, at p. 330). As Sopinka J. emphasized, “[c]ausation need not be determined [with] scientific precision” (Snell, at p. 328). A common sense approach sensitive to the realities of the situation suffices.
Causation is “essentially a practical question of fact which can best be answered by ordinary common sense rather than abstract metaphysical theory”: Snell v. Farrell,  2 SCR 311 at 328, 1990 CanLII 70, quoting Alphacell Ltd. v. Woodward,  UKHL 4,  2 All E.R. 475 at 490 (H.L.).
Undoubtedly because “law’s view of causation is less concerned with logical and philosophical considerations than with the need to produce a just result to the parties involved.” Birkholz v. RJ Gilbertson Pty Ltd. (1985), 38 SASR 121 at 130 (South Australia S.C.).
On the other hand, consider this:
Let me sum up the problem. I think that in dealing with problems of causation, courts have been too prone to resort to generalities rather than specifics. The generalities simply conceal the real reasons for the decision. … In the old days they used to talk about causa causans and causa sine qua non. … Nowadays, as we know, Latin has gone out of fashion. Sometimes, therefore, judges resort to metaphors like broken chains and such like. … The only function served by these metaphors … [is to] restate the question in more obscure language. They do not tell you how to find the answer. And the same, I am afraid, is true of simple appeals to common sense.
No one is in favour of abstract metaphysical theory. Nor is anyone against common sense. I do think, however, that judges should be encouraged to give the real reasons for their decisions. References to common sense often mean that they have not really thought them through. They are looking for the answer in generalities rather than the specifics of the legal problem which raises the question. If one does examine the specifies, it will usually be found that the answer does indeed depend upon theory; not abstract or metaphysical, but concrete, economic and political: the theory which the judge holds about the proper scope of tort or criminal liability. If this were admitted and professed, I think we would all be a step closer towards understanding what we were doing.
Then there is the role of common sense. Of course the causal requirements for liability are normally framed in accordance with common sense. But there is sometimes a tendency to appeal to common sense in order to avoid having to explain one’s reasons. It suggests that causal requirements are a matter of incommunicable judicial instinct. I do not think that this is right. It should be possible to give reasons why one form of causal relationship will do in one situation but not in another.
Fairchild v. Glenhaven Funeral Services,  UKHL 22 at para. 53,  3 All E.R. 305 per Lord Hoffman.
Or, closer to home,
Occasionally it is said that the courts, rather than being concerned with scientific or philosophical matters, adopt, and should adopt, a common sense approach to causal problems. If this means that judges have used, and should use, unsophisticated “lay” reactions to causal issues, such a view is both wrong and undesirable. Nor should the courts believe that causal issues are always easily solved. The truth is that every causal problem is susceptible to accurate analysis from a scientific or philosophical perspective, and that such analysis is on close examination mostly likely to accord with common approval.
Kozak v. Funk,  5 W.W.R. 232 at 239-240, 1997 CanLII 9801 at para. 22 (Sask. C.A. ) quoting Cooper-Stephenson, Personal Injury Damages in Canada, (Toronto: Carswell, 1981) at 639.
 Further, as this court emphasized at paras. 54 and 60 of Aristorenas [v. Comcare Health Services (2006), 83 OR (3d) 282, 2006 CanLII 33850] the robust and pragmatic approach offers a method for evaluating evidence. It is not a substitute for evidence that the defendant’s negligence caused the plaintiff’s injury; nor does it change the amount of proof required to establish causation.
 Finally, just as the robust and pragmatic approach cannot be used as a substitute for evidence, it cannot be used as a substitute for reviewing and making findings about relevant evidence.
 Put another way, the robust and pragmatic approach does not permit drawing inferences concerning either the ultimate issue of causation or links in the chain of causation without reviewing the relevant evidence and without making findings about the range of available inferences. As Rouleau J.A. pointed out at para. 63 of Aristorenas, quoting from Fairchild v. Glenhaven Funeral Services,  3 All E.R. 305 (H.L.), at para. 150, common sense cannot become a substitute for resort to the evidence:
[E]ven though it is always for the judge rather than for the expert witness to determine matters of fact, the judge must do so on the basis of the evidence, including the expert evidence. The mere application of “common sense” cannot conjure up a proper basis for inferring that an injury must have been caused in one way rather than another.
Fisher v. Victoria Hospital, 2008 ONCA 759 [words in square brackets added].
Or this, going back to Clements:
 The “but for” causation test must be applied in a robust common sense fashion. There is no need for scientific evidence of the precise contribution the defendant’s negligence made to the injury. …
 A common sense inference of “but for” causation from proof of negligence usually flows without difficulty. Evidence connecting the breach of duty to the injury suffered may permit the judge, depending on the circumstances, to infer that the defendant’s negligence probably caused the loss. ….
 Where “but for” causation is established by inference only, it is open to the defendant to argue or call evidence that the accident would have happened without the defendant’s negligence, i.e. that the negligence was not a necessary cause of the injury, which was, in any event, inevitable. ….
 This Court … [in Athey] discussed the limitations of the “but for” test and the propriety of exceptionally using a material contribution test. …. [The] Court in the end applied a robust, common sense application of the “but for” test, in accordance with Snell.
 … The law of negligence has never required scientific proof of causation; to repeat yet again, common sense inferences from the facts may suffice.
 The key finding of fact made by the trial judge was that the plaintiff had not proven causation on the basis of the “but for” test. The trial judge specifically stated, at para. 66, that the plaintiff had been “unable to prove that ‘but for’ the defendant’s breaches, she would not have been injured” … Given this finding, it would be exceedingly difficult to draw a common sense inference that those breaches caused the accident. Such inferences cannot be pulled out of thin air at the whim of the trier of fact. They must have a reliable factual foundation.
 In this case, a factual foundation that would support an inference that the overloading of the motorcycle and excessive speed caused the accident is quite simply lacking. The only evidence directly related to the issue came from the respondent’s expert, Mr. MacInnis. According to his evidence, the accident would have happened even if the motorcycle had been travelling at a lower, legal speed and without a pound of excess baggage. The trial judge evidently rejected this opinion. The fact remains, however, that no evidence was adduced regarding the exact (or even approximate) speed and weight at which the respondent would have been able to regain control of his motorcycle. The state of the evidence therefore leaves precious little room for speculating about robust common sense inferences as to the cause of the accident.
Ask yourself: if common sense is merely a process of making inferences from admissible evidence, what does is the content of this process? Does it mean anything more than making a conclusion, on the balance of probability, which could be described as a logical conclusion? If it is something more, or other than, the process of making logical conclusions, then what is that more? or other?
One can easily find authority for the proposition that a valid inference must be is more than mere speculation – that common sense is something more than mere speculation – beyond the dissent’s “The state of the evidence therefore leaves precious little room for speculating about robust common sense inferences as to the cause of the accident.” (para. 57) For example, there is Snell at 336: “Furthermore, it is not speculation but the application of common sense to draw such an inference”. The statement is in the 3rd last paragraph where the Court explains why it decided that the trial judge in Snell ought to have made the inference of factual causation on the balance of probability, hence the Court was going to make that finding. And there is this reminder adopted the Court in Hartman v. Fisette,  1 S.C.R. 248 at 243, 1976 CanLII 152: “Any other finding is speculation or conjecture as opposed to proper legal inference”.
There is an often-enough quoted (by the provincial and territorial appellate courts) attempt to explain the difference between speculation and inference. The House of Lords wrote in Caswell v. Powell Duffryn Associated Collieries Ltd.,  A.C. 152 at 169-70
Inference must be carefully distinguished from conjecture or speculation. There can be no inference unless there are objective facts from which to infer the other facts which it is sought to establish. In some cases the other facts can be inferred with as much practical certainty as if they had been actually observed. In other cases the inference does not go beyond reasonable probability. But if there are no positive proved facts from which the inference can be made, the method of inference fails and what is left is mere speculation or conjecture.
[Emphasis added]. Recent enough appellate references to Caswell (one of which is Kozak v. Funk, above) can be found at thisCanLII search. The Supreme Court of Canada has never referred to this passage in Caswell v. Powell Duffryn but the quotation from Hartman v. Fisette, a bove – “Any other finding is speculation or conjecture as opposed to proper legal inference” — is undoubtedly an acknowledgement. In any event, this CanLII search produces 7 Supreme Court of Canada statements to the same effect, including Hartman. While it is true that all but one other are criminal cases, that should not be a relevant difference beyond this much: the degree of confidence one needs in the inference one is drawing need only make the inference more likely than not. Consider some of the statements made by the Court, in the criminal law context, about what should not be done: “it was a pure invitation to the jury to resort to surmise, speculation and conjecture, as opposed to proper legal inference, against which they were cautioned” (Dunlop and Sylvester v. The Queen,  2 SCR 881 at 889, 1979 CanLII 20); or this in a portion of the judge’s charge to the jury that the Supreme Court approved: “an inference is a much stronger kind of belief than conjecture or speculation. If there are no proven facts from which an inference can be logically drawn, it is impossible to draw an inference. At best you would be speculating or guessing and that is not good enough.” (R. v. Khela,  1 SCR 104 at para. 57, 2009 SCC 4).
Or, since it seems that what we’re being told is that a common sense inference is a valid inference based on all of the admissible evidence, consider what the Supreme Court wrote in Fontaine v. British Columbia (Official Administrator),  1 SCR 424, 1998 CanLII 814 when it abolished res ipsa loquitur as a rule of law.
 As in any negligence case, the plaintiff bears the burden of proving on a balance of probabilities that negligence on the part of the defendant caused the plaintiff’s injuries. The invocation of res ipsa loquitur does not shift the burden of proof to the defendant. Rather, the effect of the application of res ipsa loquitur is as described in The Law of Evidence in Canada (1992), by John Sopinka, Sidney N. Lederman and Alan W. Bryant, at p. 81:
Res ipsa loquitur, correctly understood, means that circumstantial evidence constitutes reasonable evidence of negligence. Accordingly, the plaintiff is able to overcome a motion for a non-suit and the trial judge is required to instruct the jury on the issue of negligence. The jury may, but need not, find negligence: a permissible fact inference. If, at the conclusion of the case, it would be equally reasonable to infer negligence or no negligence, the plaintiff will lose since he or she bears the legal burden on this issue. Under this construction, the maxim is superfluous. It can be treated simply as a case of circumstantial evidence.
 Should the trier of fact choose to draw an inference of negligence from the circumstances, that will be a factor in the plaintiff’s favour. Whether that will be sufficient for the plaintiff to succeed will depend on the strength of the inference drawn and any explanation offered by the defendant to negate that inference. If the defendant produces a reasonable explanation that is as consistent with no negligence as the res ipsa loquitur inference is with negligence, this will effectively neutralize the inference of negligence and the plaintiff’s case must fail. Thus, the strength of the explanation that the defendant must provide will vary in accordance with the strength of the inference sought to be drawn by the plaintiff.
 The procedural effect of res ipsa loquitur was lucidly described by Cecil A. Wright in “Res Ipsa Loquitur” (Special Lectures of the Law Society of Upper Canada (1955), Evidence, pp. 103-36), and more recently summarized by Klar in Tort Law, supra, at pp. 423-24: …
 Whatever value res ipsa loquitur may have once provided is gone. Various attempts to apply the so-called doctrine have been more confusing than helpful. Its use has been restricted to cases where the facts permitted an inference of negligence and there was no other reasonable explanation for the accident. Given its limited use it is somewhat meaningless to refer to that use as a doctrine of law.
 It would appear that the law would be better served if the maxim was treated as expired and no longer used as a separate component in negligence actions. After all, it was nothing more than an attempt to deal with circumstantial evidence. That evidence is more sensibly dealt with by the trier of fact, who should weigh the circumstantial evidence with the direct evidence, if any, to determine whether the plaintiff has established on a balance of probabilities a prima facie case of negligence against the defendant. Once the plaintiff has done so, the defendant must present evidence negating that of the plaintiff or necessarily the plaintiff will succeed.
[Emphasis added in para. 27]
Is a common sense inference, then, anything other than a valid (whatever valid means) inference made on the balance of probability taking into account all of the evidence?
Crossing the sea, again:
The first point to emphasise is that common sense answers to questions of causation will differ according to the purpose for which the question is asked. Questions of causation often arise for the purpose of attributing responsibility to someone, for example, so as to blame him for something which has happened or to make him guilty of an offence or liable in damages. In such cases, the answer will depend upon the rule by which responsibility is being attributed.
Empress Car Co (Abertillery) Ltd. v. National Rivers Authority,  2 AC 29 at 30-31,  UKHL 5 per Lord Hoffman.
When it was first said that the sun stood still and the world turned round, the common sense of mankind declared the doctrine false; but the old saying of Vox populi, vox Dei, as every philosopher knows, cannot be trusted in science.
Not quite so long ago, Stephen Jay Gould wrote in “The Problem of Perfection, or How Can a Clam Mount a Fish on Its Rear End” in Stephen Jay Gould, Ever Since Darwin: Reflections in Natural History (New York, Norton & Co., 1979) 102 at 109 (paperback): “[c]ommon sense is a very poor guide to scientific insight for it represents cultural prejudice more often than it reflects the native honesty of a small boy before the naked emperor.”
But, it was said, once upon a time, that “[t]he lawyer cannot afford to adventure himself with philosophers in the logical and metaphysical controversies that beset the idea of cause . See Frederick Pollock, The Law of Torts (Philadelphia, The Blackstone Publishing Co., 1887) at 25). Perhaps this is because
[A] case is only an authority for what it actually decides. I entirely deny that it can be quoted for a proposition that may seem to follow logically from it. Such a mode of reasoning assumes that the law is necessarily a logical code, whereas every lawyer must acknowledge that the law is not always logical at all.
See Quinn v. Leathem,  A.C. 495 at 506 (H.L.)
Or, as a former Beatle once wrote (and sang) “Goo goo g’joob.” “I Am The Walrus”: John Lennon (Lennon and McCartney) (1967).
There’s one more item to add to the list of what was once taken as common sense: the highest courts of common law jurisdictions were infallible.