The Bellman brought a map purporting to show the elusive Snark's location: once the voyage was underway, however, the crew discovered the map was completely blank. All too often those of us in the field of drug dependence find ourselves floating on an uncharted conceptual sea, zigging and zagging in search of a common causal process. It would be as naive to suppose that all forms of heart disease have a common cause. Instead, it is more reasonable to suppose that, Just as there are similarities in the symptoms in various forms of heart disease, there are also similarities in the symptoms in various forms of drug dependence. However, in both cases one cannot expect the normal controlling mechanisms to have gone awry in precisely the same ways. It must be assumed that a relatively limited number of variables, whose weightings differ among forms of substance abuse, interact to produce the various states of dependence (see Levison's discussion, this volume).
A second problem facing the field has been the absence of a unit of analysis and a metric for assessing the control drugs exercise over the behavior of the user. It wasn't until the mid 1960's that control over Objectively measurable behavior was suggested as a criterion for assessing dependence-producing properties of drugs.
Finally, we have struggled to develop more Objective ways of assessing behavioral consequences of the drugs which are self-administered, and to provide a consistent framework within which to interpret those effects. Thus, like Janus's two faces, two oppositefacing problems of drug dependence have oriented investigators in opposite directions. Behavioral pharmacologists have treated drug