This chapter uses a theory of emotions and a theory of reasoning to elucidate psychological illnesses. The theory of emotions, which Oatley and the author developed, postulates that they are both internal communications within the brain and external communications to other individuals. There is accordingly a set of basic emotions – happiness, sadness, anger, and fear, which, as music illustrates, individuals can experience without knowing what cues them. In contrast, complex emotions, such as guilt, combine a basic emotion with a conscious access to the cognition that triggered them. The theory of reasoning is based on mental models. It postulates that intuitions (system 1) depend on mental models of what is true, and that deliberations (system 2) also represent what is false. Intuitions seek examples of hypotheses in selecting evidence, whereas deliberations also seek counterexamples. Irrelevant emotions impede reasoning, but relevant emotions enhance it. The hyper-emotion theory treats psychological illnesses as disorders in which a basic emotion occurs to a degree that is out of proportion to what is adaptive in daily life. This theory, developed with Francesco Mancini and Amelia Gangemi, has led to the discovery that patients with psychological illnesses reason no worse than healthy individuals, and outperform them in inferences related to their illnesses. Their strategies of reasoning rely on system 1 or system 2 according to their illness, and have the side effect of maintaining it. The chapter surveys evidence corroborating the theory, and assesses it implications for therapy.