What is the soul? This fundamental question finds in Aristotle an answer which may seem confusing. We expect a psychology, and he places the soul within the framework of a natural science; we are about to read a reflection on the mind or thought, and it deals with the constitutive principle of all living things - plants and animals included - to define the soul by the nutritive, sensitive, appetitive and intellective functions. Form of a living being, the soul is inseparable from the notion of body of which it becomes the principle of life.