 If the point of tort law were to achieve an efficient allocation of resources, and if such efficiency could be achieved only by discovering the “true costs” of each activity when such costs are defined in terms of that activity’s harmful effects, then “cause” as used in tort liability rules should mean whatever the metaphysics of causation tells us the word means. For on this theory it is the harmful effects that an activity really causes that are the true costs for that activity; and this rationale thus demands a robust use of some metaphysical view about causation.