Hot-shot appellate counsel will tell you to lead with your best argument.
Appellate judges will tell you to get to the point immediately.
Assume, for arguments' sake – the assumption is easier because you're assuming that the law is what the law is – that the principles for causation are the same for omissions as they are for commissions: for negligence which is a failure to act as well as negligence which is an action.
Let's do a small thought experiment. You won't need a pen or paper, or any electronic equivalent.
Assume that two or more people each have a duty to water Simon F's plants. Neither did. Simon's plants died. Simon's plants would have lived if any of the people had watered the plants.
Assume that two or more drivers each failed to stop at a stop sign. All the vehicles struck Simon's vehicle. Simon was injured. Each impact was severe enough to cause the injuries that Simon suffered even if none of the others had occurred.
Assume Simon's injury is nothing more than a broken ankle. However, Simon has a high blood pressure problem if he doesn't take his medication. He is on the way to the hospital because it's 2 a.m. Sunday morning in Toronto, the next day is Labour day, his prescription has run out, his pharmacy will be closed, as will be the walk-in clinic he frequents.
The paramedics who take him to the hospital take his blood pressure and find it within acceptable limits given his comminuted ankle fracture. For whatever reason, either they don't ask him, or he doesn't tell them, about a blood pressure problem. The emergency nurse forgets to check his blood pressure. The resident forgets to check Simon's blood pressure. The emergency room physician forgets to check Simon's blood pressure. Simon doesn't mention it. By the time he gets to the hospital, he's in extreme pain. He dies of a problem related to his blood pressure, not the broken ankle, as he's being wheeled towards the X-ray room. If anyone had checked his blood pressure at any time after his arrival at the hospital, he'd not have died.
Another version of Simon F is employed as a technician for a nuclear reactor used to generate power. Remarkably, so is a version of Simon C. Both operate separate fail-safe shut-down devices. Call it a deadman switch.The reactor starts to melt down. Both Simons fail to activate their own switch. Either switch would have stopped the melt down. (I won't complicate things by asking you to assume that, unbeknownst to Simon F, his switch had failed earlier that day, just after he'd tested it, because the maintenance crew had failed to service it the night before. That's not something Simon F ought to have known because another team failed to service his status read out, so it told him the switch was operating properly and had been serviced.)
Given Clements v. Clements 2012 SCC 32, para. 46 what's the applicable test for causation – but for or material contribution – in all of these instances?
 The foregoing discussion leads me to the following conclusions as to the present state of the law in Canada:
(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.
(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.
If you agree that 46(2) – material contribution – seems to apply then ask yourself this question: can we call a test which seemingly apples to every instance of causation involving multiple tortfeasors and multiple omissions a test which applies "exceptionally"?
Not if we accept the commonly understood meaning of "exceptionally". Maybe even the common sense meaning.
Would you agree that, paraphrasing Resurfice, at para. 19:
To accept this conclusion is to do away with the “but for” test altogether, in cases of multiple tortfeasors and multiple omissions, given that there is more than one potential cause in all litigated cases of negligence of this sort. If the Supreme Court's reasons in this regard are endorsed, the only conclusion that could be drawn is that the default test for causation is now the material contribution test. This is inconsistent with the Supreme Court's judgments in Snell, Athey, Walker Estate, Blackwater, Fullowka, Resurfice and, curiously, Clements.
But what other conclusion is there in all instances of individually sufficient omissions? Suffiicient, here, means together with all of the other necessary facts, other than the negligence of the other tortfeasors.
Remember, you can't say that Clements applies only to commission (misfeasance) but not omission (non-feasance). (Well … you CAN say that. You'll just be wrong.)
Of course, if this argument is correct, the tortfeasor caught by it won't have to worry about the uncertainty of common sense. I'm sure that'll be a small comfort.
Curious minds, etc.