Rare threats: Possibilistic, probabilistic, and Precautionary analysis

The focus of this paper is on four approaches to decision making about man-made systems which have some potential for catastrophic failure despite the fact that these systems are engineered and managed for safety. The four approaches are categorized in terms of the theory of possibility. If an adverse event is only partially possible and not at all necessary (that is, its complement is completely possible), the appropriate approach is possibilistic decision making. If any event is completely possible and not at all necessary, this is the domain of standard probability theory. If an adverse event is completely possible and somewhat necessary (that is, its complement is only partially possible), the Precautionary Principle is appropriate. Finally, if an adverse event is only partially possible and partially necessary, this is a sign that there is so little information about the propensity of the event to occur that the event should be treated as a speculative threat.