Rothstein J. wrote for the Supreme Court  in F.H. v. McDougall, [2008] 3 SCR 41 at para. 46, 2008 SCC 53

If a responsible judge finds for the plaintiff, it must be accepted that the evidence was sufficiently clear, convincing and cogent to that judge that the plaintiff satisfied the balance of probabilities test.

He also wrote a few paragraphs later at para. 54:

[54]    Rowles J.A. was correct that failure by a trial judge to apply the correct standard of proof in assessing evidence would constitute an error of law. The question is how such failure may be apparent in the reasons of a trial judge. Obviously in the remote example of a trial judge expressly stating an incorrect standard of proof, it will be presumed that the incorrect standard was applied. Where the trial judge expressly states the correct standard of proof, it will be presumed that it was applied. Where the trial judge does not express a particular standard of proof, it will also be presumed that the correct standard was applied:

 Trial judges are presumed to know the law with which they work day in and day out.

 (R. v. Burns, [1994] 1 S.C.R. 656, at p. 664, per McLachlin J. (as she then was))

 Whether the correct standard was expressly stated or not, the presumption of correct application will apply unless it can be demonstrated by the analysis conducted that the incorrect standard was applied. However, in determining whether the correct standard has indeed been applied, an appellate court must take care not to substitute its own view of the facts for that of the trial judge

In Clark v. Kouba, 2012 BCSC 1607 the trial judge wrote:

CAUSATION

[59]        In Athey v. Leonati, [1996] 3 S.C.R. 458, [1996] S.C.J. No. 102, the court has established the general principles for the establishment of causation commencing at paras. 13 – 20:

[13]        Causation is established where the plaintiff proves to the civil standard on a balance of probabilities that the defendant caused or contributed to the injury: Snell v. Farrell, [1990] 2 S.C.R. 311; McGhee v. National Coal Board, [1972] 3 All E.R. 1008 (H.L.).

[14]        The general, but not conclusive, test for causation is the “but for” test, which requires the plaintiff to show that the injury would not have occurred but for the negligence of the defendant: Horsley v. MacLaren,  [1972] S.C.R. 441.

[15]        The “but for” test is unworkable in some circumstances, so the courts have recognized that causation is established where the defendant’s negligence “materially contributed” to the occurrence of the injury: Myers v. Peel County Board of Education;  [1981] 2 S.C.R. 21, Bonnington Castings, Ltd. v. Wardlaw, [1956] 1 All E.R. 615 (H.L.); McGhee v. National Coal Board, supra. A contributing factor is material if it falls outside the de minimis range: Bonnington Castings, Ltd. v. Wardlaw, supra; see also R. v. Pinske  (1988), 30 B.C.L.R. (2d) 114 (B.C.C.A.), aff’d  [1989] 2 S.C.R. 979.

[16]        In Snell v. Farrell, supra, this Court recently confirmed that the plaintiff must prove that the defendant’s tortious conduct caused or contributed to the plaintiff’s injury. The causation test is not to be applied too rigidly. Causation need not be determined by scientific precision; as Lord Salmon stated in Alphacell Ltd. v. Woodward, [1972] 2 All E.R. 475, at p. 490, and as was quoted by Sopinka J. at p. 328, it is “essentially a practical question of fact which can best be answered by ordinary common sense”. Although the burden of proof remains with the plaintiff, in some circumstances an inference of causation may be drawn from the evidence without positive scientific proof.

[17]        It is not now necessary, nor has it ever been, for the plaintiff to establish that the defendant’s negligence was the sole cause of the injury. There will frequently be a myriad of other background events which were necessary preconditions to the injury occurring. To borrow an example from Professor Fleming (The Law of Torts (8th ed. 1992) at p. 193), a “fire ignited in a wastepaper basket is … caused not only by the dropping of a lighted match, but also by the presence of combustible material and oxygen, a failure of the cleaner to empty the basket and so forth”. As long as a defendant is part of the cause of an injury, the defendant is liable, even though his act alone was not enough to create the injury. There is no basis for a reduction of liability because of the existence of other preconditions: defendants remain liable for all injuries caused or contributed to by their negligence.

[18]        This proposition has long been established in the jurisprudence. Lord Reid stated in McGhee v. National Coal Board, supra, at p. 1010:

       It has always been the law that a pursuer succeeds if he can shew that fault of the defender caused or materially contributed to his injury. There may have been two separate causes but it is enough if one of the causes arose from fault of the defender. The pursuer does not have to prove that this cause would of itself have been enough to cause him injury.

[19]        The law does not excuse a defendant from liability merely because other causal factors for which he is not responsible also helped produce the harm: Fleming, supra, at p. 200. It is sufficient if the defendant’s negligence was a cause of the harm: School Division of Assiniboine South, No. 3 v. Greater Winnipeg Gas Co., [1971] 4 W.W.R. 746 (Man. C.A.), at p. 753, aff’d  [1973] 6 W.W.R. 765 (S.C.C.), [1973] S.C.R. vi; Ken Cooper-Stephenson, Personal Injury Damages in Canada (2nd ed. 1996), at p. 748.

[20]        This position is entrenched in our law and there is no reason at present to depart from it. If the law permitted apportionment between tortious causes and non-tortious causes, a plaintiff could recover 100 percent of his or her loss only when the defendant’s negligence was the sole cause of the injuries. Since most events are the result of a complex set of causes, there will frequently be non-tortious causes contributing to the injury. Defendants could frequently and easily identify non-tortious contributing causes, so plaintiffs would rarely receive full compensation even after proving that the defendant caused the injury. This would be contrary to established principles and the essential purpose of tort law, which is to restore the plaintiff to the position he or she would have enjoyed but for the negligence of the defendant.

[emphasis in original]

[60]        The Supreme Court of Canada re-affirmed these principles in Clements v. Clements,  2012 SCC 32, 346 D.L.R. (4^th) 577. Applying these principles to this case, I am satisfied, based on all of the evidence, that the plaintiff suffered soft tissue injuries to her neck, shoulders, mid- and upper-back which were caused by the accident. These injuries have resulted in chronic pain as described in detail in Dr. Armstrong’s report. Ms. Clark has suffered headaches, emotional and cognitive distress and sleep disorder as a result of the persistent pain. Overall I found Dr. Armstrong’s report and evidence of great assistance in assessing the impact of the accident on Ms. Clark’s life.

…

[69]        In summary, the evidence of the plaintiff, her experts and the temporal connection between the accident and the injuries lead me to conclude that the injuries Ms. Clark described were caused by the accident and are chronic in nature

Based on para. 60, what principles did the trial judge actually apply?  (The answer isn’t elsewhere in the reasons.)

If we gave the reasons to a first year law student, after he or she has taken torts, could he or she identify what test for factual causation the trial judge applied? He or she would know what test the judge was supposed to have applied, but that isn’t my question.

( I suppose, had I had more time a few weeks ago, I could have attempted to find out. I gave a lecture (about causation) to a class of first year law students. They clapped when it was over. I’m going to assume that wasn’t just because the lecture was over and it what I had reviewed wasn’t going to be on their mid-term exam. Also, their professor mentioned that most of them were paying attention and not surfing.)

Could a member of the public reading the decision understand why the judge made the decision she did, beyond understanding that the judge believed the plaintiff and preferred the evidence of the plaintiff’s physicians?

We – meaning the part of the profession who practices in the area – can make informed assumptions as to the principles the trial judge must have had in mind, and then decide from the evidence recounted in the reasons,  but is that enough?

If we assume (as we are required to, absent some indication to the contrary) that the judge knew the applicable law,  that the evidence was as the trial judge recounts, that it was admissible, and that the trial judge did not make any relevant errors accepting it – something none of us can decide without reading the transcript – then the decision is correct in law (meaning that it cannot be successfully appealed).

But is that enough?