This thesis analyses the method, purpose, and results of Aristotle's treatment of a select number of Presocratic and Platonic theses about the soul within the context of De Anima 1. Contrary to a prevalent interpretation of De Anima 1 which sees Aristotle's treatment of his predecessors' psychological views as dialectical, I argue that Aristotle treats his predecessors as having offered potentially viable hypotheses about the nature of the soul, and that these hypotheses are treated as attempts to explain the soul's basic powers. I further show that, in order to test the explanatory limits of these theories, Aristotle uses a version of the scientific method of inquiry advertised in the Prior and Posterior Analytics, which consists in setting out the basic psychological phenomena which psychology should explain, and then testing the extent to which his predecessors' definitions of the soul are able to accomplish this task. This thesis argues that, by demonstrating where his predecessors' first principles fall short, Aristotle is able to make theoretical progress towards establishing his own 'hylomorphic' first principle of soul, that soul is the 'first fulfilment of a natural instrumental body', and towards the idea that soul operates in the body of living beings as a formal, final, and efficient cause.