According to the combat model of the soul, reason and "passion" (as the 18th-century philosophers like to put it) compete for control of our actions. We act morally and rationally when reason wins. This conventional picture makes very little sense, however. How is an agent supposed to choose between reason and passion, when reason itself is the faculty of choice? Or if the agent does not choose between them, but is simply overwhelmed by one of them, why do we attribute the resulting action to the agent at all? (It's no accident that we speak of being overwhelmed by passion, but not of being overwhelmed by reason). For that matter—if they serve the same function—providing motives to act—how is the difference between reason and passion any different from the difference between two different passions? According to the constitutional model, which I derive from Plato's famous comparison between the city-state and the soul, reason and passion are not competitors for the same role, but rather serve different functions—roughly speaking, passion proposes and reason disposes. Our emotions, desires, and passions suggest things we might do, but reason decides. If they are in conflict, passion is getting out of line and trying to usurp reason's function, which is the government of the self.
I prefer this model because it makes better sense of our psychic economy. Reason and passion do different things; they are not just sources of different kinds of motives.