  There is some dispute about which of the psychic abilities mentioned by Aristotle in De Anima qualify as full-fledged or autonomous faculties. He evidently accepts the three already mentioned as centrally important. Indeed, he is willing to demarcate a hierarchy of life in terms of them. Even so, he also discusses two other capacities, imagination (De Anima iii 3) and desire (De Anima iii 9 and 10), and appeals to them in both his account of thinking and his philosophy of action. He does little, however, to characterize either in any intrinsic way. He evidently regards imagination as a sort of subordinate faculty, integrated in various ways with the faculties of nutrition, perception, and thought. Desire is still more complex. Despite its not occurring without the sensory faculty (De Anima iii 7 412a12–14), desire seems in the end elevated to a full capacity, primarily because of its role in the explanation of purposive behavior. His discussions of imagination and desire raise interesting questions about how Aristotle views the various capacities of soul as integrating into unified forms. They also raise questions along with his discussions of the other faculties as to how Aristotle conceives the unity of the whole soul. Some scholars seem content to characterize an Aristotelian soul as a set or sum of capacities, whereas Aristotle himself evidently demands a non-aggregative form of unity (De Anima ii 3 414b28–32, cf. iii 9 432a–b6).