  In mechanizing the concept of living thing, Descartes did not deny the distinction between living and nonliving, but he did redraw the line between ensouled and unensouled beings. In his view, among earthly beings only humans have souls. He thus equated soul with mind: souls account for intellection and volition, including conscious sensory experiences, conscious experience of images, and consciously experienced memories. Descartes regarded nonhuman animals as machines, devoid of mind and consciousness, and hence lacking in sentience.  (Although Descartes' followers understood him to have denied all feeling to animals, some recent scholars question this interpretation; on this controversy, see Cottingham 1998 and Hatfield 2008.) Consequently, Descartes was required to explain all of the powers that Aristotelians had ascribed to the vegetative and sensitive soul by means of purely material and mechanistic processes (11:202). These mechanistic explanations extended, then, not merely to nutrition, growth, and reproduction,  but also to the functions of the external and internal senses, including the ability of nonhuman animals to respond via their sense organs in a situationally appropriate manner: to approach things that are beneficial to their body (including food) and to avoid danger (as the sheep avoids the wolf).