The negligent tortfeasor's mental state is irrelevant. P must show that D's conduct, viewed as of the time it occurred , without benefit of hindsight, imposed an unreasonable risk of harm. D does not escape liability merely because she intended to behave carefully or thought she was behaving carefully. Thus if D has a physical disability , the standard for negligence is what a reasonable person with that physical disability would have done.
P is blind and is struck while crossing the street using a cane.
If the issue is whether P was contributorily negligent, the issue will be whether a blind person would have crossed the street in that manner. If D is more stupid, or more careless, than an ordinary person, this will not be a defense. A child is held to the level of conduct of a reasonable person of that age and experience , not that of an adult. But where a child engages in a potentially dangerous activity normally pursued only by adults , she will be held to the standard of care that a reasonable adult doing that activity would exercise. Courts generally allow evidence as to custom for the purpose of showing presence or absence of reasonable care.