Not every unintentional act that causes injury is negligent. If you brake to a stop when you see a child dart out in front of your car, and if the noise from your tires gives someone in a nearby house a heart attack, you have not acted negligently toward the person in the house. The purpose of the negligence standard is to protect others against the risk of injury that foreseeably would ensue from unreasonably dangerous conduct.
Given the infinite variety of human circumstances and conduct, no general statement of a reasonable standard of care is possible. Nevertheless, the law has tried to encapsulate it in the form of the famous standard of “the reasonable man.” This fictitious person “of ordinary prudence” is the model that juries are instructed to compare defendants with in assessing whether those defendants have acted negligently. Analysis of this mythical personage has baffled several generations of commentators. How much knowledge must he have of events in the community, of technology, of cause and effect? With what physical attributes, courage, or wisdom is this nonexistent person supposedly endowed? If the defendant is a person with specialized knowledge, like a doctor or an automobile designer, must the jury also treat the “reasonable man” as having this knowledge, even though the average person in the community will not? (Answer: in most cases, yes.)
Despite the many difficulties, the concept of the reasonable man is one on which most negligence cases ultimately turn. If a defendant has acted “unreasonably under the circumstances” and his conduct posed an unreasonable risk of injury, then he is liable for injury caused by his conduct. Perhaps in most instances, it is not difficult to divine what the reasonable man would do. The reasonable man stops for traffic lights and always drives at reasonable speeds, does not throw baseballs through windows, performs surgical operations according to the average standards of the medical profession, ensures that the floors of his grocery store are kept free of fluids that would cause a patron to slip and fall, takes proper precautions to avoid spillage of oil from his supertanker, and so on. The "reasonable man" standard imposes hindsight on the decisions and actions of people in society; the circumstances of life are such that courts may sometimes impose a standard of due care that many people might not find reasonable.