Rationality is a subject that can be studied in many different ways. In this paper, I will consider the role of rationality within the framework of the philosophy of psychology, or to be more precise, in the framework of the commonsense psychological explanation. Commonsense psychology explains behavior in terms of beliefs and desires. We attribute these mental states to persons and then based on those mental states, we interpret their behavior. In order to do so successfully, it is important to assume that people are rational agents. That means that their reasoning and behavior are led by some normative principles. Therefore it is often thought that commonsense psychological explanations cannot be reduced to cognitive-neuroscientific explanations. Commonsense psychological explanations are based on reasons for actions, while cognitive neuroscience relies on causal mechanistic explanations. Usually, it is thought that these principles of rationality are grounded in logic and probability theory. However, people often do not follow such norms of rationality and mistakes are often made. The question is whether this means that the rationality assumption is false and what implications this fact has for commonsense psychological explanations. The main claim of this thesis is that rationality is constitutive for commonsense psychological explanation. However, this means that it should be reconsidered what are the correct principles of rationality that are constitutive for commonsense psychological explanations.