 Prediction always involves an element of defeasibility. If one predicts what will, or what would, under some hypothesis, happen, one must presume that there are no unknown factors that might interfere with those factors and conditions that are known. Any prediction can be upset by such unanticipated interventions. Prediction thus proceeds from the assumption that the situation as modeled constitutes a closed world: that nothing outside that situation could intrude in time to upset one’s predictions. In addition, we seem to presume that any factor that is not known to be causally relevant is in fact causally irrelevant, since we are constantly encountering new factors and novel combinations of factors, and it is impossible to verify their causal irrelevance in advance. This closed-world assumption is one of the principal motivations for McCarthy’s logic of circumscription (McCarthy 1982; McCarthy 1986).