Aristotle, to me, was perhaps the greatest thinker of the whole of ancient Hellas. His ideas have largely been proven incorrect by modern science and psychology but they are no less intriguing for it. One of those theories is the theory of the three types of souls. As Aristotle himself wrote:

"The knowledge of the soul admittedly contributes greatly to the advance of truth in general, and, above all, to our understanding of Nature, for the soul is in some sense the principle of animal life. [...] To attain any assured knowledge about the soul is one of the most difficult things in the world."

The soul, according to Aristotle, is a grounding principle of sorts. It is the realization of life. The soul is the one thing that enables a body to engage in the necessary activities of life and they build upon one another. the more parts of the soul a being possesses, the more evolved and developed he is. the three types of soul are the nutritive soul, the sensible soul, and the rational soul.

The nutritive soul is the first and most widely shared among all living things. For it can be said that anything that takes in nutrition, grows from this nutrition, and eventually decays over time has a soul. Plants, for example, possess the nurative soul solely while it is one of two or three parts of the soul possessed by animals and humans. the nutritive soul is what urges any creature to protect itself whenever possible, but also to produce offspring in any form because it's own life is finite.

The sensible soul, or the soul of perception, is the part of the soul that allows us to perceive the world around us. It encompasses the senses but also allows us to remember things that happened to us, experience pain and pleasure, and have appetites and desires. Most animals and all humans possess the sensible soul while plants to not. Of course, not all animals have the same abilities of perception. those who solely possess sense organs for a single sense can potentially not be actualized by the sensible soul and are more like plants, possessing only the nutritive soul. A cricket, for example, or mollusks.

Aristotle believed that animals and humans both possess the sensible soul. However, he asks the question if animals have the capacity for belief. Belief would seem to imply conviction. Conviction would seem to imply that a creature was persuaded, because one can not be convinced of something without being persuaded in some way. Finally, persuasion would seem to imply a rational function of measuring possibilities and drawing conclusions, a function that Aristotle believed animals did not possess.

The rational soul belongs to man alone. The rational soul is that by virtue of which we possess the capacity for rational thought. Aristotle divides rational thought into two groups. The first is the passive intellect. It is the part of our mind that collects information and stores it for later use. This is almost an extension of the sensible soul in that it allows us to act upon the information gathered by that part of the soul.

The active intellect is the part that allows us to engage in the actual process of thinking. It allows us to take our sensory input, combine it with our memories and skills and apply it to our betterment. Aristotle also believed that the active intellect was responsible for our ability to consider abstract concepts that we have never perceived. Through active intellect, philosophy becomes possible and it is this ability that distinguishes humans from animals.

Modern science has disproven much of Aristotle’s ideas on the soul. It states that there is no nutritive, sensible, or rational soul. All three levels of the Aristotelian soul can be explained by nutritive and perception organs. And the rational soul is merely the rapid firing of neurons in our brain. And yet, it makes sense. It's the understanding of the human mind through experience alone. It is man explainign the world through it's own eyes--and yes, by way of the three parts of the soul. It's the theory, applied. It's absolutely beautiful to me solely for that.