  The soul is an important and central issue in the ER, related as it is to Ibn Daud’s treatment of the aforementioned key-concepts of Aristotelian natural philosophy on the one hand, and to his discussions of cosmology, prophecy and ethics on the other. About one fifth of the book is devoted to an investigation of the soul, its nature and faculties, its relation to the body and its afterlife (ER W 20–41; S 54b–116b). Ibn Daud infers the existence of something like the soul from the fact that different natural bodies are characterized by various actions, which can only be explained by positing the existence of an immaterial principle that is added to their corporeality and from which these actions derive. Following Aristotle, Ibn Daud defines the soul as the perfection of a natural organic body that possesses life potentially. Elaborating on the distinction between soul and accident, he maintains that the soul is a substance (in the sense of form), refuting the view of the “physicians” that it is a mixture, thereby dismissing all other theories according to which the soul is an accident. In contradistinction to earlier Jewish philosophers of neo-Platonic orientation, Ibn Daud emphasizes the unity of the soul.