  In these, as in countless other cases, the explanation of animal action, human and non-human alike, easily and unreflectively appeals to desire. This is why Aristotle does not end his De Anima with a discussion of mind. Instead, after discussing mind, he notes that all animals are capable of locomotion, only to deny that any one of the faculties of the soul so far considered (viz. nutrition, perception, or mind) can account for desire-initiated movement. Although he had initially identified only these three faculties of soul (De Anima ii 2, 413b12), Aristotle now notes that something must explain the fact that animals engage in goal-directed behavior in order to achieve their conscious and unconscious goals. The wanted explanation cannot, he urges, be found somehow in the nutritive faculty, since plants, as living beings, have that power of soul, but do not move themselves around in pursuit of their goals; nor is it due to perception, since even some animals have this faculty without ever moving themselves at all, in any way (Aristotle evidently has in mind sponges, oysters, and certain testacea, Historia Animalium i 1, 487b6–9; viii 1 588b12; Partibus Animalium iv 5, 681b34, 683c8); nor again can it be a product of mind, since insofar as it is contemplative, mind does not focus upon objects likely to issue in directives for action, and insofar as it does commend action, mind is not of itself sufficient to engender motion, but instead relies upon appetite (De Anima iii 9, 432b14–33a5). Indeed, using the same form of reasoning, that a faculty cannot account for purposive action if its activity is insufficient to initiate motion, Aristotle initially concludes that even desire itself (orexis) cannot be responsible for action. After all, continent people, unlike those who are completely and virtuously moderate, have depraved desires but do not, precisely because they are continent, ever act upon them (De Anima iii 9 433a6–8; cf. Nicomachean Ethics i 13, 1102b26). So their desires are insufficient for action. Consequently, he concludes, desire alone, considered as a single faculty, cannot explain purposive action, at least not completely.