Recent experimental evidence (Damasio, 1994), (Bechara et al., 1997) suggests that emotions, rather than being sand in the machinery of rationality, are a condition of possibility of rational decision. The main goal of this paper is to present a model of the functional role of emotions in decision making. General ideas about the role of emotions in cognition, first presented in (Simon, 1967), are applied to the problem of rational decision. The model that thus arises is empirically testable. New experimental tests based on existing data are suggested and possible applications in knowledge representation are discussed.