I believe that what I've written below is a good enough summary, for now, of what practitioners in Canada's common law jurisdictions need to know about the effect of Clements v Clements, 2012 SCC 32 on the manner in which causation is to be proved in negligence actions. (For those who don't know, Quebec is a civil law jurisdiction; all others are common law.)
These propositions are written for the Canadian lawyer whose knowledge of the relevant Canadian law is such that a Superior Court (or equivalent) judge would consider that lawyer competent to prosecute or defend an "ordinary" personal injury or property damage action. As such, they presume a certain level of knowledge.
Comments are now on for a limited purpose. I will attempt to clarify any of these propositions if the manner in which I have stated the proposition is not sufficiently clear, bearing in mind what I have said is the premise upon which the propositions are written. That means I will not attempt, in this forum, to justify any of the statements. If you disagree, you disagree. Ultimately, none of us, here, have the final say – even in this life.
What follows is accurate enough for daily practice, except in exceptional cases (which by definition shouldn't be part of daily practice). The bold sentences are likely all you need for now.
In what follows, "factual causation" describes the historical relationship found to exist, on the balance of probability, between the negligence of a defendant and the injury of a plaintiff. "Causation", by itself, refers to all of the requirements of the causation requirement of the cause of action, in a particular case.
The overall point is: Clements should have no substantive effect on day to day practice (except in British Columbia which now has to fall in line with the rest of common law Canada).
1. Judges are to determine if factual causation exists – if there is a factual causation relationship between a defendant's negligence and a plaintiff's injury – by applying the but-for test "in a robust common sense fashion" as outlined in Snell v. Farrell.
The but-for test is the only approved method of establishing factual causation on the balance of probability.
2. The but-for test is always to be applied using the robust common sense approach, regardless of whether expert evidence is or is not called on the factual causation issue.
British Columbia lawyers should pay special attention. The SCC just overruled, without mentioning that it was doing so, the line of BCCA authority that began with Moore v. Castlegar & District Hospital, 1998 CanLII 4906 (BCCA) and was recently reiterated in Ediger v. Johnston, 2011 BCCA 253. That line held that the Snell common sense approach can't be used where expert evidence is adduced on the factual causation issue. BC lawyers will know that there are cases between the two I've listed that extend the proposition to instances where only one side calls the expert evidence.
Clements did not use "pragmatic" as in "robust, pragmatic, common sense" in para. 9*. I have no idea if that's significant.
*Corrections in red made July 14, 2010. Clements does use "pragmatic" but not together with "common sense".
It's in paragraph 46(1)
46(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.
This time, what's missing is "common sense". I conclude that the absence of "pragmatic" in para. 9 is not significant; nor is the absence of "common sense" in para. 46(1). One might say the Court has defined a robust and pragmatic approach to be a common sense approach, or a common sense approach to be a robust and pragmatic approach. I'm not sure how that's helpful. Others may disagree. (The causation discussion in the latest edition – the 10th – of Fleming, The Law of Torts, can be read to suggest that the common sense approach amounts to answering the question the Australian way.)
3. Lawyers and judges have been told to stop talking about, and attempting to resort to, any flavour of Resurfice material contribution in every day practice.
This isn't a substantive difference in doctrine. It is a practical difference to the extent it means that, in most cases, trial judges will now not have to attempt to explain the law on an issue which won't apply. (Of course, one's view on whether that's a difference which matters may be influenced by how much one enjoys reading the law portions of reasons for judgment.)
Based on Clements, Resurfice material contribution should not have been applied in Leslie v. S & B Apartments Ltd., 2011 NSSC 48. That's for our East coast readers.
4. The Athey material contribution test is now formally defunct as a method for establishing factual causation on the balance of probability in tort actions, even if the SCC did not expressly say that.
Perhaps the Court thought it wasn't necessary to formally declare that view of Athey material contribution dead, since the Court seems to have said it was never correct in the first place. Still …
Whatever Athey material contribution meant before Resurfice (2007) and before Clements, it was (then) understood as an alternative method for establishing factual causation on the balance of probability. You don't have to take my word for that. You don't have to look up any cases. Just look at every mainstream Canadian tort treatise. Look at every collection of cases and materials used in any Canadian law school. Those of you who still have your first year law school notes (if you started law school in 1997 through June 2012) might look at them.
5. For now, Resurfice material contribution applies only in the situations described by Clements in para. 39, where the Court provided an explanation of the meaning of Resurfice's "impossible to establish factual causation on the balance of probability using the but-for test." Qualifying impossibility requires two or more wrongdoers each of whose conduct could, in fact, have been a cause even if the conduct of all of the other wrongdoers had not occurred.
The fact that the conduct of the other tortfeasors did occur is what makes it impossible to validly use the but-for test to determine whether the negligence of any of the tortfeasors was factual cause.
Does this sound like the "circular causation" description, in the British Columbia Court of Appeal's explanation – an explanation that the SCC said was not required? See Clements, para. 45. Well … yes, so long as one understands 'circular causation" to mean logical impossibility. I think what the SCC was trying to say is that logical impossibility is not the definition of Resurfice impossibility.
A problem with the two or more tortfeasors requirement is that it is inconsistent with the second Resurfice example of impossibility which does not require two tortfeasors (or wrongdoers).
This is where I get to mention (again) Lord Halsbury's reminder in Quinn v. Leathem  A.C. 495 at 506 (H.L.). (But not yet Alice in Wonderland. That comes later.) The wonder of modern technology means you don't have to get out of your seat to find Lord Halsbury's reminder. Just click on the link and read the first paragraph of the case. Keep in mind the point of item 7, below.
6. Resurfice material contribution, if applied, does not produce a finding of factual causation on the balance of probability. It allows a plaintiff to satisfy the causation requirements of the cause of action without establishing factual causation on the balance of probability.
See Clements at para. 14.
 “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. As set out by Smith J.A. in MacDonald v. Goertz, 2009 BCCA 358 (CanLII), 2009 BCCA 358, 275 B.C.A.C. 68, at para. 17,
. . . “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” … That is because to deny liability “would offend basic notions of fairness and justice”: Hanke v. Resurfice Corp., para. 25.
For those who care, the Court preferred the British Columbia Court of Appeal's view of the doctrine over the apparent Ontario Court of Appeal view. The ONCA had said it is a test for factual causation. Frazer v. Haukioja, 2010 ONCA 249 at para. 41: "Factual causation can be established in two ways: the 'but for' test and the 'material contribution' test."
Some judges in the ONCA seem to think that Resurfice material contribution is nothing more than what Athey material contribution was, but with more restrictive conditions. In an odd, "O. Henryish" way, they might have been proven right: but only if it is the case that the SCC is now asserting (see para. 15, and see item 7) that Athey material contribution should never have been understood to be a method of establishing factual causation on the balance of probability
7. Justice Hugo Jackson wrote about the United States Supreme Court in Brown v. Allen (1953), 344 U.S. 443 at 540: "we are not final because we are infallible, but we are infallible only because we are final."
In 2003, Lord Hope of Craighead (of the Appellate Committee of the U.K. House of Lords – now the U.K. Supreme Court) wrote in "Decision Overrruled – Facing Up to Judicial Fallibility (2003), 14 K.C.L.J. 121 at 122, quoting from a 1932 Sottish judgment:
The House of Lords has a perfect legal mind. Learned Lords may come or go, but the House of Lords never makes a mistake. That the House of Lords should make a mistake is as unthinkable as that Colonel Bogey should be bunkered twice and take 8 to the hole. Occasionally, to some of us two decisions of the House of Lords may seem inconsistent. But that is only a seeming. It is our frail vision that is at fault.
Lord Hope remarked the passage had "perhaps just a trace of sarcasm". That passage may remind some Antony's
The fault, dear Brutus, is not in our stars,
But in ourselves, that we are underlings.
in Shakespeare, Julius Caesar, Act I, scene ii, lines 140-141.
But, of course, in the professional, hierarchical, sense, we are but small peckers obliged to obey. (See South Side Woodwork v. R.C. Contracting, 1989 CanLII 3384 (AB QB)).
8. Clements' comments about multiple tortfeasor situations and Resurfice contribution cannot be understood to mean that instances of "indivisible" injuries are now to be handled under the Resurfice material contribution test rather than the but-for test.
The brief version of the doctrinal explanation why includes: (1) a finding of "indivisible" injury amounts to a legal conclusion that the causal candidates will be treated as cumulatively necessary causes of the injury; and (2) given what Resurfice material contribution means, applying that doctrine to successive accident cases would produce a legion (not just a a host) of difficulties.
The practical explanation, in addition to (2), is that treating successive accident cases as instances to which Resurfice material contribution applies would make that test the default test for those instances and, if one is logically consistent (but see item 5) the default test everywhere else.
That is because every injury is the result of multiple necessary conditions. Some of the other necessary conditions may be wrongful, once actionable, events. Some may not be. The effect, in law, of these other conditions on the causal status of the defendant's negligence isn't a question of factual causation. It might be an issue that triggers a remoteness (proximate cause) rule - a rule that limits the extent of liability. Or it might trigger a damages assessment rule. Again, a rule that limits the extent of liability. These issues likely underlie the discussion in para. 21 of Clements.
9. We should assume that the Clements commandments will apply to all tort actions.
But, see the commentary in item 5.
In any event, it seems a safe guess that the Clements analysis will spread to other areas of law which have causation requirements, to the extent permitted by the judiciary's views on the principles governing those other areas.
Put less obliquely, that means that, for now, nothing has changed in those areas of law – for example, workers' compensation, or motor vehicle accident benefits — where Athey material contribution, as it was understood before Resurfice and before Clements, has been held to be the meaning of the causation concept in the governing statute or regulations.
I'm sure the SCC was aware of Twain's admonition to not argue with those who buy ink by the gallon but was also aware it doesn't apply to the Court, for the reason that Humpty Dumpty gave Alice (in Through the Looking Glass, c. VI) in concluding their argument about the meaning of words. "The question is," said Humpty Dumpty, "which is to be master – that‘s all."
In this area of a Canadian lawyer's professional life, the Supreme Court of Canada is master (until a legislature competent to do so enacts otherwise).
10. There is no 11. Yet.