Negligence is determined by a standard of care that a relevant prudent person would undertake, rather than the results that such a theoretical person would seek to attain or avoid.
Although Canada initially inherited the reasonable person standard from England in Vaughn v. Menlove, 1837 132 ER 490, it has developed significantly since that time. It as not, however, made exceptions for mental illness as in Vaughn, but has created a number of other exceptions, most notably for experts and professionals.
Every medical practitioner must bring to his task a reasonable degree of skill and knowledge and must exercise a reasonable degree of care. He is bound to exercise that degree of care and skill which could reasonably be expected of a normal, prudent practitioner of the same experience and standing, and if he holds himself out as a specialist, a higher degree of skill is required of him than of one who does not profess to be so qualified by special training and ability.
I do not believe that the standard of care required of a medical practitioner has been more clearly or succinctly stated than by Lord Hewart C.J. in Rex v. Bateman (1925), 41 T.L.R. 557 at 559: “If a person holds himself out as possessing special skill and knowledge and he is consulted, as possessing such skill and knowledge, by or on behalf of a patient, he owes a duty to the patient to use due caution in undertaking the treatment. If he accepts the responsibility and undertakes the treatment and the patient submits to his direction and treatment accordingly, he owes a duty to the patient to use diligence, care, knowledge, skill and caution in administering the treatment … The law requires a fair and reasonable standard of care and competence.”
This is not a standard of perfection, as this would constitute a guarantee of a specific result that the professional would attain or avoid. This was explained further by the Court in a more recent case in St-Jean v. Mercier, as follows,
53 The correct inquiry to be made in assessing whether a professional committed a fault is indeed to ask whether the defendant behaved as would a reasonably prudent and diligent fellow professional in the same circumstances (J.-L. Baudouin and P. Deslauriers, La responsabilité civile (5th ed. 1998), at p. 850). To ask, as the principal question in the general inquiry, whether a specific positive act or an instance of omission constitutes a fault is to collapse the inquiry and may confuse the issue. What must be asked is whether that act or omission would be acceptable behaviour for a reasonably prudent and diligent professional in the same circumstances. The erroneous approach runs the risk of focussing on the result rather than the means. Professionals have an obligation of means, not an obligation of result.
[emphasis in the original]
The Court recently granted an appeal in Armstrong v. Ward, upholding the dissent Court of Appeal’s decision in Armstrong v. Royal Victoria Hospital. The majority set aside the finding of liability by the trial judge in the medical negligence case involving a colectomy surgery, where the plaintiff was awarded $1.3 million. The judgment was based on a conclusion that the physician breached the standard of care by creating an injury to the ureter, which resulted in the loss of the plaintiff’s kidney.
The majority at the Court of Appeal found that the trial judge had imposed an improper standard of care when measuring the physician’s liability. They found it to be an error of law to hold the physician to the goal of a prudent surgeon when conducting the operation, rather than the means that a prudent surgeon would use to attain that goal,
 A trial judge who is prepared to proceed on the basis that only negligence could cause the relevant injury is obliged to consider and rule out non-negligent causes. Only if this is done, can the trial judge properly use success as the standard of care. In determining whether this is so, the burden is not on the defendant to raise potential non-negligent causes with evidence, nor is it improper speculation for a trial judge to consider potential non-negligent causes that are open on the evidence but that the plaintiff has failed to address. A plaintiff whose liability theory is that only negligence could have caused the injury in question is obliged to demonstrate that this is so, and the trial judge is required to accept this before finding liability. That did not occur in this case.
 Nor can liability properly be grounded in the low risk of injury identified by the trial judge. It is a logical error to infer that since an adverse result is improbable, a defendant was negligent in causing that adverse result. Negligence needs to be proved in each specific case, unless it is established that the kind of injury in question can only occur through negligence. Yet, as explained, the trial judge never found this to be so.
In contrast, the dissent at the Court of Appeal, which the Supreme Court upheld with a simple oral judgment, found that the trial judge made no reversible error, and did not conflate the standard of care with causation. The case was largely about factual causation of the injury to the ureter,
 I have spent some time outlining the key disputed questions at trial because they inform an understanding of the trial judge’s reasons and the expert evidence. In contrast to the conflicting causation opinions, there was little, if any, dispute between the experts about the standard of care… Nor was this a case about misadventure or about circumstances beyond the control of a surgeon. Indeed, there was no evidence in this trial to suggest that a competent surgeon using reasonable care while operating to remove a benign colon would be unable to take the necessary and important step of staying more than two millimetres away from the ureter. The evidence was to the contrary—it should have been routine.
The trial judge did not hold the physician to a higher standard than what could be reasonably expected of a prudent and reasonable general surgeon conducting the same procedure in the circumstances of the case. The negligence in this case was not a goal in the procedure, namely staying away from the ureter, but rather a step in the procedure. A failure to properly follow that step would indeed breach the standard of care, as substantiated by the expert evidence on the procedure.
Although the majority of the Court of Appeal concluded that the the physician took the appropriate steps to identify and protect the ureter, they concluded an error of law because of the trial judge’s conclusion that a normal, prudent surgeon would undertake this step in a manner that avoids proximity to the ureter.
The dissent instead characterized the decision as a failure of the physician to properly undertake the necessary steps as a finding of fact. None of the evidence at trial suggested that a surgeon exercising reasonable care could not stay away from the ureter,
 Each of the experts testified that staying away from the ureter was part of the standard of care, and a necessary step. None of the witnesses posited a situation where a competent surgeon, in the context of surgery on a normal abdomen, could accidentally come too close to the ureter.
 Although all of the expert witnesses agreed that it would be more difficult to avoid the ureter if the anatomy was abnormal or if there were masses, no one suggested that a competent surgeon operating in the “ideal” circumstances that Ms. Armstrong presented, could with reasonable care have come too close to the ureter, let alone suggested how this could have happened… Given the considerable amount of time spent on causation, it is telling that the experts did not opine on “non-negligent” ways that Dr. Ward could have come within two millimetres of Ms. Armstrong’s ureter. It was simply not the focus of the evidence in this case.
The trial judge was therefore satisfied by the evidence of what was otherwise considered a basic surgery that it would be a breach of the standard of care to come close to the ureter. This was not backwards reasoning, or a conclusion finding of negligence based on a low probability of risk.
Despite otherwise being a rather straightforward surgical procedure, part of the difficulty with medical negligence cases is that in some cases a trial judge is required to determine how an injury occurred, before even assessing the standard of care,
 …While Bafaro tells us that the “but for” question of causation cannot be answered until the standard of care analysis is done, Meringolo, Grass and Kennedy tell us that in some cases it will be necessary to determine “what happened” before analyzing whether there has been a breach of the standard of care.
The Court’s granting of the appeal in Amstrong may renew a focus on figuring out “what happened,” which for many medical negligence plaintiffs is one of the primary motivations for litigation in the first place.