On the Possibility of an Event

This paper introduces a concept of {\it possibility} that may be applied to real-world events that are viewed as having {\it prerequisites} that must be satisfied in order for the event to occur and/or {\it constraints} that may impede or prevent the event's occurrence.  The core idea is to compute the possibility of the event as a function of the probabilities that the prerequisites are satisfied and the constraints are not, and where this function computes possibility values of logical combinations of prerequisites and constraints according to the principles of fuzzy logic.  Thus, this notion of possibility is a hybrid of fuzzy logic and probability theory.  It has the advantage that it provides an objective procedure for determining the possibility of an event insofar as the probabilities of the prerequisites and constraints may be determined by statistical samplings.  This version of possibility would appropriately be applied to problems of planning.  When there are multiple plans available for achieving a goal, this theory can be used to determine which plan is most possible, i.e., easiest or most feasible to complete.  It is speculated that this model of reasoning more correctly captures normal human reasoning about plans.  This may be contrasted with the pure use of probability theory, which can be used to determine which plan is most likely to succeed, but is not typically employed by humans in their everyday decision making.