In the preceding section, we gave an informal overview of the notion of agency. In this section, we turn our attention to the theory of such agents, and in particular, to formal theories. We regard an agent theory as a specification for an agent; agent theorists develop formalisms for representing the properties of agents, and using these formalisms, try to develop theories that capture desirable properies of agents. Our starting point is the notion of an agent as an entity `which appears to be the subject of beliefs, desires, etc.' [Seel, 1989]. The philosopher Dennett has coined the term intentional system to denote such systems.