It is hypothesized that pleasure serves as the common currency in the tradeoffs between clashing motivations. The displeasure of frustrating one motivation being accepted for the sake of a larger pleasure obtained in satisfying another one. This thesis will be presented step by step starting with the role of pleasure in situations involving only one motivation for a behavior serving only one physiological aim. The second step will be devoted to situations where two motivations clash for behavior that serves physiological aims. Relations exist between pleasure and usefulness and between displeasure and harm or danger.
Ethologists differentiate between goal and cost.
To the Physiologist a behavior is optimal when it leads to homeostasis. The experiments reported above were limited to sensory pleasure and to conflicts of motivations with clear physiological implications. One may question whether it is possible to extend the conclusions to other domains than biology. The notion of behavioral common path is especially enlightening to answer that question. Therefore, the brain, responsible for the behavioral response, must rank priorities and determine tradeoffs in the decisions concerned with allocating time among competing behaviors.