For more than 1800 years it has been supposed that Aristotle viewed the soul as the entelechy of the visible body which is 'equipped with organs'. This book argues that in actual fact he saw the soul as the entelechy of a natural body 'that serves as its instrument'. This correction puts paid to W. Jaeger's hypothesis of a three-phase development in Aristotle. The author of this book defends the unity of Aristotle's philosophy of living nature in De anima, in the biological treatises, and in the lost dialogues. Aristotle should therefore be regarded as the author of the notion of the 'vehicle of the soul' and of a 'non-Platonic' dualism. The current understanding of his influence on Hellenistic philosophy needs to change accordingly.