  Although willing to provide a common account of the soul in these general terms, Aristotle devotes most of his energy in De Anima to detailed investigations of the soul’s individual capacities or faculties, which he first lists as nutrition, perception, and mind, with perception receiving the lion’s share of attention. He later also introduces desire, evidently as a discrete faculty on par with those initially introduced. The broadest is nutrition, which is shared by all natural living organisms; animals have perception in addition; and among natural organisms humans alone have mind. Aristotle maintains that various kinds of souls, nutritive, perceptual, and intellectual, form a kind of hierarchy. Any creature with reason will also have perception; any creature with perception will also have the ability to take on nutrition and to reproduce; but the converse does not hold. Thus, plants show up with only the nutritive soul, animals have both perceptual and nutritive faculties, and humans have all three. The reasons why this should be so are broadly teleological. In brief, every living creature as such grows, reaches maturity, and declines. Without a nutritive capacity, these activities would be impossible (De Anima iii 12, 434a22–434b18; cf. De Partibus Animalium iv 10, 687a24–690a10; Metaphysics xii 10, 1075a16–25). So, Aristotle concludes, psychology must investigate not only perceiving and thinking, but also nutrition.