Tort law relies heavily on the concept of reasonable care, and specifically the reasonable person standard. Negligence is typically described as a failure to act with the prudence of a reasonable person. The reasonable person standard, we will see in this chapter, is objective, in the sense that it does not depend on the particular preferences or idiosyncratic psychological features of the defendant before the court. And although it is objective, it is not easily summarized in the form of a simple cost-benefit test. The reasonable person standard incorporates the typical individual's ability to make long-term plans that might affect the risks he imposes on others and to make tradeoffs that affect those risks.

Recall that in Brown v. Kendall (Chapter 4), Chief Justice Shaw defined reasonable care as the care that a prudent and cautious man would take to guard against probable danger. In many of the early negligence cases, this is as specific as it gets in terms of a definition of reasonable care. However, even this thin formulation is sufficient to convey some important ideas. The reasonable person, it appears, will take probable losses to others into account and will modify his conduct to avoid causing harm to others.

Most of the early formulations of the reasonable person standard do not explain just how much weight the reasonable person would put on the danger to others. Would the reasonable person treat the danger to others with the same level of concern as he would treat danger to himself, or would he treat it with less concern? The difficulty in specifying precisely how much weight should be put on risks to others suggests that the reasonable person should treat them as equals and put just as much weight on probable harms to others, in his calculus of precaution, as he would put on probable harms to himself. Indeed, it would seem contradictory for the reasonable person to discount probable harms to others, because he values his own interests more than theirs, and at the same time demand that those others not discount the harms their conduct might impose on him.