Negligence is the breach of a duty of care. In general, the law allows me to mind my own business. Thus, if I happen to see someone else's child about to drown in a pool, ordinarily I do not owe a legal duty to anyone to try to save it. But sometimes the law requires me to be my brother's keeper. This happens, for example, when the circumstances are such that I owe him a duty of care; and I am negligent if I breach it. I owe him such a duty if a ''diligens paterfamilias'', that notional epitome of reasonable prudence, in the position in which I am in (), would—

(a) foresee the possibility of harm occurring to him; and

(b) take steps to guard against its occurrence.

Foreseeability of harm to a person, whether he be a specific individual or one of a category, is usually not a difficult question, but when ought I to guard against it? It depends upon the circumstances in each particular case, and it is neither necessary nor desirable to attempt a formulation which would cover all cases. For the purposes of the present case it is sufficient to say, by way of general approach, that if I launch a potentially dangerous undertaking involving the foreseeable possibility of harm to another, the circumstances may be such that I cannot reasonably shrug my shoulders in unconcern but have certain responsibilities in the matter—the duty of care.〔373E-H.〕