How do Ontario courts decide what a reasonable person would have done in a negligence case?

Once a duty of care is established, Ontario courts measure whether it was breached against an objective "reasonable person" standard - not what the specific defendant subjectively believed was safe, but what an ordinary, reasonably prudent person would have done in the same circumstances. The court weighs several factors together: how likely the harm was, how serious it could be if it happened, the cost and practicality of taking precautions against it, and any social value in the activity being carried out. A small risk of serious injury can still require precautions if guarding against it was cheap and easy; an activity with real social value may tolerate a higher risk before it counts as unreasonable.

Professionals and specialists, such as doctors, engineers, and contractors, are judged against a reasonable person with their training - a higher, skill-specific standard, usually established through expert evidence about accepted practice. Common industry practice is relevant but not conclusive; an entire industry can still be found negligent if its usual way of doing things is itself unreasonably risky. Because this is a fact-heavy, comparative exercise, the strongest evidence in these cases tends to be documentary - inspection records, maintenance logs, industry standards, and expert opinion - rather than argument about the law itself.