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Aristotle

Aristotle (384–322 BCE) defined the soul, or Psūchê (ψυχή), as the " first actuality " of a naturally organized body, and argued against its separate existence from the physical body. In Aristotle's view, the primary activity, or full actualization, of a living thing constitutes its soul. For example, the full actualization of an eye, as an independent organism, is to see (its purpose or final cause). Another example is that the full actualization of a human being would be living a fully functional human life in accordance with reason (which he considered to be a faculty unique to humanity). For Aristotle, the soul is the organization of the form and matter of a natural being which allows it to strive for its full actualization. This organization between form and matter is necessary for any activity, or functionality, to be possible in a natural being. Using an artifact (non-natural being) as an example, a house is a building for human habituation but for a house to be actualized requires the material, such as wood, nails, or bricks necessary for its actuality (i.e., being a fully functional house); however, this does not imply that a house has a soul. In regards to artifacts, the source of motion that is required for their full actualization is outside of themselves (for example, a builder builds a house). In natural beings, this source of motion is contained within the being itself.

Aristotle addressed 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 humans 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; it is argued 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. Furthermore, Aristotle states that the soul helps humans find the truth, and understanding the true purpose or role of the soul is extremely difficult.