  Aristotle's theory, as it is presented primarily in the De Anima (for a complete account,  see Aristotle's Psychology),  comes very close to providing a comprehensive, fully developed account of the soul in all its aspects and functions, an account that articulates the ways in which all of the vital functions of all animate organisms are related to the soul. In doing so, the theory comes very close to offering a comprehensive answer to a question that arises from the ordinary Greek notion of soul, namely how precisely it is that the soul, which is agreed to be in some way or other responsible for a variety of things living creatures (especially humans) do and experience, also is the distinguishing mark of the animate. According to Aristotle's theory, a soul is a particular kind of nature, a principle that accounts for change and rest in the particular case of living bodies, i.e. plants, nonhuman animals and human beings. The relation between soul and body, on Aristotle's view, is also an instance of the more general relation between form and matter: thus an ensouled, living body is a particular kind of in-formed matter. Slightly simplifying things by limiting ourselves to the sublunary world (cf. De Anima 2.2, 413a32; 2.3, 415a9), we can describe the theory as furnishing a unified explanatory framework within which all vital functions alike, from metabolism to reasoning, are treated as functions performed by natural organisms of suitable structure and complexity. The soul of an animate organism, in this framework, is nothing other than its system of active abilities to perform the vital functions that organisms of its kind naturally perform, so that when an organism engages in the relevant activities (e.g., nutrition, movement or thought) it does so in virtue of the system of abilities that is its soul. 