To determine what an emotion is, is a matter of conceptualization--namely, understanding and explicitly encoding the concept of emotion as people use it in everyday life. This is a notoriously difficult problem. The purpose of this paper is to give a formal representation of the concept of emotion. The view underlying our approach is that primitive concepts are constructed out of the observation of behavioral regularities. The behavioral regularities required for characterizing emotion concepts, together with a level at which the regularities are described, are examined. We argue that the concepts of emotion and desire are one and the same, and we define the concept of desire (or action tendency) in a formal language. We also address the concept of emotion intensity.