  In both perception and thinking, animal souls are in some ways active and in some ways passive. Although both mind and the sensory faculty receive their correlative forms when perceiving or thinking, neither is wholly passive in its defining activity. Perception involves discrimination, while thinking involves selective attending and abstraction, both activities, in the sense that each requires more than mere passive receptivity. Still, the sorts of activity required for cognition and perception do not explain in any obvious way another central fact about human beings and other animals: animals propel themselves through space in pursuit of objects they desire. Even in his first characterizations of soul in De Anima, Aristotle is alive to the widely held conviction that the soul is implicated in motion (De Animai 2, 405b11; i 5 409b19–24). Of course, this is a natural connection for him to make, given that every animate being, that is, every being with a soul, has within it a principle of motion and rest. So, it seems deeply characteristic of living systems that they are able to move themselves in ways likely to result in their survival and flourishing. Animals move themselves, however, in a distinctive way: animals desire things, with the result that desire is centrally implicated in all manner of animal action. Why did ostrich run from the tiger? Because, one says easily, it desired to survive and so engaged in avoidance behavior. Why did the human being drive to the opera and sit quietly in her seat? Because, it seems, she desired to hear the music and to observe the spectacle.