Soul

===Aristotle===
[[File:Aristotelian Soul.png|thumb|upright=1.5|The structure of the souls of plants, animals, and humans, according to [[Aristotle]], with ''Bios'', ''Zoê'', and ''Psūchê'']]
{{further|Aristotle's biology}}

Aristotle (384–322 BCE) defined the soul, or ''Psūchê'' (ψυχή), as the "[[first actuality]]" of a naturally organized body,<ref>{{cite book|last=Aristotle|title=On The Soul|page=412b5}}</ref> 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]]).<ref>{{cite book |last=Aristotle |title=Physics |at=Book VIII, Chapter 5, pp. 256a5–22}}</ref> 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).<ref>{{cite book |last=Aristotle |title=Nicomachean Ethics |at=Book I, Chapter 7, pp. 1098a7–17}}</ref> 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 (wood, nails, bricks, etc.) 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.<ref>{{cite book |last=Aristotle |title=Physics |at=Book III, Chapter 1, pp. 201a10–25}}</ref> 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.<ref>{{cite book |last=Aristotle |title=On The Soul |at=Book III, Chapter 5, pp. 430a24–25}}</ref> 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 [[Aristotelianism|Aristotelian]] text contains this specific point, and this part of ''De Anima'' is obscure.<ref>{{cite book |chapter-url=http://plato.stanford.edu/entries/aristotle-psychology/active-mind.html |title=Aristotle's Psychology |chapter=supplement: The Active Mind of De Anima iii 5) |access-date=2013-12-12 |last=Shields |first=Christopher |author-link=Christopher Shields |year=2011 |publisher=The Stanford Encyclopedia of Philosophy}}</ref> Further, Aristotle states that the soul helps humans find the truth, and understanding the true purpose or role of the soul is extremely difficult.<ref>{{cite book |title=Introduction to Aristotle |last=Smith |first=J. S. (Trans) |publisher=University of Chicago Press |year=1973 |location=Chicago |pages=155–59}}</ref>