The source of life: activity, capacity, and biology in Aristotle's account of soul
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Aristotle discusses the nature of soul in De Anima, defining it as the "form of a natural body having life potentially" or "first actuality of a natural, instrumental body." I argue that these definitions characterize soul as the capacity for the activity of life. In chapter one I examine key terminology from Aristotle’s account of soul: the terms used to discuss soul, life, and the vital functions. In particular, the soul and life terminology must be kept separate, as must the terms referring to vital capacities and those referring to vital activities. In chapter two I use these terminological distinctions to trace Aristotle’s arguments for his definition of soul, contending that they begin by positing life as the vital activities and soul as the cause of life. From that beginning, Aristotle twice argues for a definition of soul, in De Anima 2.1 and 2.2. In the transition between the two arguments Aristotle says that the first is sketched in outline and that a proper definition shows the cause. While this is usually taken to mean that Aristotle prefers the second definition, I argue that the definitions reached are the same. In chapter three I argue that Aristotle’s definitions of soul state that it is the capacity for life. He defines it as a first actuality, and upon examination this phrase means that it is a capacity. He also defines it as a form and calls form an actuality, but I explain that due to the relativity of actuality and potentiality, it is permissible to view form as a capacity as well. In chapter four I reconcile the general account of soul as a capacity with Aristotle’s discussions of a particular kind of soul, examining what he says in De Anima and his biological works about the most fundamental kind—the nutritive. Aristotle locates nutritive soul in the heart and says that it is responsible for the size of an organism, but this fits with nutritive soul also being the capacity of an organism to nourish itself. I also discuss why Aristotle says the body is the instrument of soul.