Aristotle defines the soul or hé psyché as “the entelechy of a natural body endowed with the capacity of life” or as “the first entelechy of a natural organic body (psyché estin entelecheia hé prooté soomatos physikou dynamei zoon echontos toiouton de, ho an hé organikon or entelecheia hé prooté soomatos physikou organikou). Being the actuality or entelechy of the body, the soul is at the same time form, principle of movement, and end. The body is for the soul, and every organ has its purpose, that purpose being an activity.
The composite substance is a natural body endowed with life, the principle of this life being called the soul. The body cannot be the soul, for the body is not life but what has life. The body then is matter to the soul, while the soul is as form or actuality to the body. the soul is the entelechy or act of the body that possesses life in potency—“potentiality of life.”
Thus the soul as the principle of living body is a) the source of motion,b) its final cause, c) the real substance (i.e., the formal cause) of animated bodies.
Aristotle distinguishes different types of the soul, whereby the higher presupposes the lower.
1) The lowest form of soul is nutritive or vegetative soul, to threptikon, which exercises the activities of assimilation and reproduction. It found not only in plants, but also in animals; yet it can exist b y itself (without the other type of soul), as it does in plants.
2) Animal soul. Because the nutrition is necessary for the preservation of life, Aristotle reveals the necessity of touch in order to find the food, and the taste to articulate the food from the rest. The other senses are not necessary for the animal, Aristotle argues, they serve the well-being of the animal.
2-1) Sensation or sensitive soul (h €