Only in British Columbia, eh?
(but not yet any other province ending in “a”).
Or, how to conflate factual causation – what caused what – with the separate limitation of liability question(s) of remoteness – is there any reason, in law, that this particular factual cause should not be treated as a legal factual cause so that the court may then consider if the plaintiff has satisfied the rest of the requirements of the cause of action.
A British Columbia trial judge wrote, recently:
 Accordingly, to satisfy the “but for” test, a plaintiff must show that the defendant caused his or her injury by proving on a balance of probabilities that the defendant’s conduct was the sole cause of his or her injury, or that there was a substantial connection between the defendant’s actions and the injury, beyond the de minimus level.
Perry v. Vargas, 2012 BCSC 1925 at para. 105
Don’t shoot the trial judge who accurately summarized (and cited) British Columbia Court of Appeal authority since Resurfice and Clements in para. 104.
However, since the Supreme Court of Canada still has the last word, the quoted proposition can’t be taken to mean that necessity isn’t the substance of the but-for test in British Columbia tort law. (Clements, Resurfice, Athey, Snell). The trial judge recognized this at para. 100.
 The test for causation is the “but for” test: the plaintiff must show on a balance of probabilities that “but for” the negligence of the defendant the injury would not have occurred. …
But, since necessity is the cornerstone of but-for, substantial connection means something else. So, para. 105 has to be understood to mean necessity (but-for) and substantial connection (something more). That something else, to mean anything in law, must be remoteness.
Do we have authority for this in some other province ending in “a”? As it happens, yes: Alberta, in Nattrass v. Weber, 2010 ABCA 64. Slatter J.A., writing for the majority, stated at para. 47: “In Athey, de minimis or “non‑material contribution” is an exception to liability where several causes contribute to the damage. It could be described as a type of de minimis defence or limit on liability.” This is probably a reference to para. 41.3 of Athey:
If the accidents alone could have been a sufficient cause, and the pre-existing back condition alone could have been a sufficient cause, then it is unclear which was the cause-in-fact of the disc herniation. The trial judge must determine, on a balance of probabilities, whether the defendant’s negligence materially contributed to the injury.