Purposeful behaviour is a characteristic feature of most species of animals. Some instances of such behaviour are generally classified as actions. A basic intuition is that only human beings are capable of action, and that it is in virtue of the power of reason that human beings have this capacity. However, it is open to debate as to how action requires reason, and what sense of reason is predicated in the first place. The purpose of the project is to make a thorough investigation of the first attempt to articulate the rationality of action in a systematic way, which is found in Aristotle, and which still shapes the discussion in this field. A common assumption is that, according to Aristotle, actions are rational in virtue of being deliberate, i.e. are preceded by practical reasoning. However, this answer faces the problem that not all actions seem to be deliberate, and that Aristotle himself mentions actions that are non-deliberate. A choice is required: either Aristotle does not embrace the idea that actions are rational in nature, or he finds the source of rationality elsewhere. The project makes a case for the latter option. The thesis is that reason's own desire, i.e. wish, has a bearing on all actions, whether deliberate or not.