On account of both its worth and its accuracy, Aristotle at the beginning of the De Anima (I. I. 402Ai-7) ranks knowledge of soul among the topics that enjoy the highest priorities. He regards it as making great contribution towards the acquisition of truth in general, and in particular towards the philosophy of nature. Against this background he may be expected to show keen interest in working out a correct and penetrating definition of soul. According to the norms of his own logic, the nature of a thing as expressed in its definition is the starting point of every demonstration in scientific procedure;¹ and where, as in the present case, the subject is introduced as offering an especially high degree of accuracy, the enticement to spend sufficient time on the basic definition involved could hardly help but make itself felt.