The transformation of the science of the soul between 1260 and 1360. Aristotle's highly influential work on the soul, entitled De anima, formed part of the core curriculum of medieval universities and was discussed intensively. It covers a range of topics in philosophical psychology, such as the relationship between mind and body and the nature of abstract thought. However, there is a key difference in scope between the socalled ‘science of the soul', based on Aristotle, and modern philosophical psychology. This book starts from a basic premise accepted by all medieval commentators, namely that the science of the soul studies not just human beings but all living beings. As such, its methodology and approach must also apply to plants and animals. The Science of the Soul discusses how philosophers, from Thomas Aquinas to Pierre d'Ailly, dealt with the difficult task of giving a unified account of life and traces the various stages in the transformation of the science of the soul between 1260 and 1360. The emerging picture is that of a gradual disruption of the unified approach to the soul, which will ultimately lead to the emergence of psychology as a separate discipline.