themselves (for example, a builder builds a house). In natural beings, this source of motion is contained within the being itself. Aristotle elaborates on this point when he addresses the faculties of the soul. The various faculties of the soul, such as nutrition, movement (peculiar to animals), reason (peculiar to humans), sensation (special, common, and incidental) and so forth, when exercised, constitute the "second" actuality, or fulfillment, of the capacity to be alive. For example, someone who falls asleep, as opposed to someone who falls dead, can wake up and live their life, while the latter can no longer do so. Aristotle identified three hierarchical levels of natural beings: plants, animals, and people, having three different degrees of soul: Bios (life), Zoë (animate life), and Psuchë (self-conscious life). For these groups, he identified three corresponding levels of soul, or biological activity: the nutritive activity of growth, sustenance and reproduction which all life shares (Bios); the self-willed motive activity and sensory faculties, which only animals and people have in common (Zoë); and finally "reason", of which people alone are capable (Pseuchë). Aristotle's discussion of the soul is in his work, De Anima (On the Soul). Although mostly seen as opposing Plato in regard to the immortality of the soul, a controversy can be found in relation to the fifth chapter of the third book: in this text both interpretations can be argued for, soul as a whole can be deemed mortal, and a part called "active intellect" or "active mind" is immortal and eternal. Advocates exist for both sides of the controversy, but it has been understood that there will be permanent disagreement about its final conclusions, as no other Aristotelian text contains this specific point, and this part of De Anima is obscure. Further, Aristotle states that the soul helps humans find the truth, and understanding the true purpose or role of the soul is extremely difficult. Avicenna and Ibn al-Nafis Following Aristotle, Avicenna (Ibn Sina) and Ibn al-Nafis, an Arab physician, further elaborated upon the Aristotelian understanding of the soul and developed their own theories on the soul. They both made a distinction between the soul and the spirit, and the Avicennian doctrine on the nature of the soul was influential among the Scholastics. Some of Avicenna's views on the soul include the idea that the immortality of the soul is a consequence of its nature, and not a