A small conundrum involving the SCC’s decisions in Clements and McDougall and the standard of proof as it applies to the impossibility requirement in the material contribution to risk test.
An orthodox statement of the requirements of the negligence cause of action is: duty, breach of the standard of care; causation and damages.
In civil cases, the standard of proof for past events is the balance of probability: F.H. v. McDougall, 2008 SCC 53 at para. 44, 2008 SCC 53
 Put another way, it would seem incongruous for a judge to conclude that it was more likely than not that an event occurred, but not sufficiently likely to some unspecified standard and therefore that it did not occur. As Lord Hoffmann explained in In re B at para. 2:If a legal rule requires a fact to be proved (a “fact in issue”), a judge or jury must decide whether or not it happened. There is no room for a finding that it might have happened. The law operates a binary system in which the only values are zero and one. The fact either happened or it did not. If the tribunal is left in doubt, the doubt is resolved by a rule that one party or the other carries the burden of proof. If the party who bears the burden of proof fails to discharge it, a value of zero is returned and the fact is treated as not having happened. If he does discharge it, a value of one is returned and the fact is treated as having happened.In my view, the only practical way in which to reach a factual conclusion in a civil case is to decide whether it is more likely than not that the event occurred.
 The foregoing discussion leads me to the following conclusions as to the present state of the law in Canada:
(1) [omitted – the but-for part] ….
(2) Exceptionally, a plaintiff may succeed by showing that the defendant’s conduct materially contributed to risk of the plaintiff’s injury, where (a) the plaintiff has established that her loss would not have occurred “but for” the negligence of two or more tortfeasors, each possibly in fact responsible for the loss; and (b) the plaintiff, through no fault of her own, is unable to show that any one of the possible tortfeasors in fact was the necessary or “but for” cause of her injury, because each can point to one another as the possible “but for” cause of the injury, defeating a finding of causation on a balance of probabilities against anyone.
Clements does not refer to McDougall nor discuss the standard of proof. That should mean that mean that the standard of proof for causation under the Clements material contribution to risk is the balance of probability.
If we are to take 46(2) for what it says, the events that have to be proven to exist on the balance of probability amount to the existence of at least two alternative negligent events which could be but-for causes where the evidence is such that it (1) permits only one of the events actually to have been the cause and (2) the evidence is not sufficient to allow a valid choice amongst the events.
In other words, the test described in para. 46(2) of Clements requires proof on the balance of probability that two or more events exist where the evidence does not permit the judge or jury to conclude validly that any one of the events, or any combination of the events sufficient to be a but-for cause, was more likely or not to have occurred.
But, if the plaintiff cannot establish that any of these events, alone or in combination, is more likely or not to have occurred, then the plaintiff cannot satisfy McDougall if McDougall applies.
If so, then Clements material contribution to risk has to be seen as an exception to McDougall which, in theory, seems to make sense since Clements material contribution causation is based on probabiliy not possibility.
(Given that Clements was decided after McDougall, we no longer have to consider what it might mean that McDougall was decided after Resurfice, but did not mention Resurfice. Even though it was Resurfice that (seemingly) created the material contribution to risk test. Why do I say seemingly? Because Clements seems to suggest that Athey material contribution ought always to have been understood to be a material contribution to risk test based on possibility, not probability. But that makes things more complicated since McDougall doesn’t mention Athey, either.)
Can one reconcile McDougall and Clements? Recall the concluding sentence of  in McDougall.
In my view, the only practical way in which to reach a factual conclusion in a civil case is to decide whether it is more likely than not that the event occurred.
Perhaps the problem is that the causal relationship is not an event like other events: such as the fact of the injury, the fact of the conduct.
More on this another day.