The usual approach in determining when negligence causes an injury is to apply what is commonly called the ‘but for’ test: ‘but for the action or inaction of the defendant, the plaintiff would not have been injured’.
 A substantial connection between the injury and the defendant’s conduct is also required. It ensures that the injuries will not be attributable to the defendant if they are “… due to factors unconnected to the defendant and not the fault of anyone” …
 Resurfice Corp. v Hanke, 2007 SCC 7 at paras 21-22,  1 SCR 333; Clements v Clements, 2012 SCC 32 at para 46, 346 DLR (4th) 577.
 Ibid at para 23.
Let’s ignore the conflict between the statements in the first and second sentences of . One such conflict is this: if there is any connection between the defendant’s conduct and the injury, then the injury is not “unconnected” to the defendant. That the connection might not be a basis for imposing liability is not a question of factual causation.
Instead, let’s consider the problems created by  if it was intended to modify  and, so, to deal with proof of factual causation. Did the trial judge mean to assert that a necessary cause is not a factual cause unless it is also a substantial cause? If so, that would be (ahem) necessarily contrary to the “plain” meaning of “necessary”, apparently contrary to para. 8 of Clements
 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.
which the trial judge did not reference. The reference to para. 46 of Clements is to para. 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.
and, one should not forget, explicitly contrary to everybody’s (?) seminal case, Athey v Leonati,  3 SCR 458, 1996 CanLII 183 at para. 41.2
2. If it was necessary to have both the accidents and the pre-existing back condition for the herniation to occur, then causation is proven, since the herniation would not have occurred but for the accidents. Even if the accidents played a minor role, the defendant would be fully liable because the accidents were still a necessary contributing cause.
The underlining is in the original.
While the “substantial connection” phrase was repeated in Clements, it was not in the context of somehow relating the use in para. 23 of Resurfice to the meaning of “necessary” in the but-or test. The discussion, in Clements, appears in paragraphs 20-23, in the context of the court’s discussion of the development of a test for legal causation other than the but-for test.
In any event, I think most plaintiffs lawyers would NOW be surprised to be told that, to establish factual causation, they have to go beyond “mere” necessity, whatever “mere” means in this context, and establish “substantial” necessity, whatever “substantial” means in this context.
In case one anyone wonders – the reasons in Malton are quite long – I saw nothing in my review of what I thought to be portions relevant to this discussion to suggest the trial judge meant para. 493 to refer to “proximate cause”, i.e., remoteness arguments.