Next, a successful plaintiff must prove that the defendant failed to act in a reasonable manner. Using the previous example, if Dave operates his car in an unreasonable manner, it is said that he has "breached" his duty of care toward Sally. Thus, Dave has been negligent. The determining factor as to whether conduct is reasonable is based on whether a prudent person could foresee that certain conduct would pose a risk to others. For example, a prudent person could foresee that driving 90 miles per hour in a 30 mile per hour zone would pose a risk to others and would easily be characterized as unreasonable behavior.

Lastly, the plaintiff must show that he or she suffered actual damages or loss as a result of the unreasonable behavior. If Dave drives 90 miles per hour down the street he may be in violation of speeding laws but Sally does not have a claim against Dave until she is injured as a result of Dave's unreasonable driving. However, in the opening example, Sally carelessly walked out into the street before Dave struck her. In this case, Dave is likely to assert contributory negligence as a defense against his liability for Sally's injuries.